September roster expansion no longer means that clubhouses are crowded with armies of new faces. Teams now can increase their rosters only from 26 to 28 for the final month.
Still, there are opportunities for impact prospects to get their first chance at the Majors, following in a long tradition. A number of the game's greatest -- from Hall of Famers to some of today's top stars -- have made their big league debuts as September callups.
Here's a breakdown of each club's best September callup.
AMERICAN LEAGUE EAST
Blue Jays: Roy Halladay
Debut: Sept. 20, 1998 vs. Devil Rays
Halladay's Hall of Fame career began as a September callup. He allowed three runs (two earned) over five innings in his MLB debut against the Devil Rays, but it was the second outing that left a lasting memory. Halladay was one out away from pitching the second no-hitter in franchise history when outfielder Bobby Higginson broke up the bid with a solo home run in the bottom of the ninth. It took a few more years for Halladay to become a mainstay in the rotation, but he ended up becoming one of the Blue Jays' all-time greats.
Orioles: Brooks Robinson
Debut: Sept. 17, 1955 vs. Senators
Robinson made his debut at Memorial Stadium against the Washington Senators, batting sixth and going 2-for-4 with an RBI. Thus started a Hall of Fame career for the 18-time All-Star, who spent his entire career with Baltimore.
Rays: David Price
Debut: Sept. 14, 2008 vs. Yankees
The No. 1 overall pick in the 2007 MLB Draft posted a 2.30 ERA in 109 2/3 Minor League innings that year as a starter, but he became a relief weapon for Tampa Bay in September as it clinched its first AL East title. Price famously closed out Game 7 of the 2008 ALCS against the Red Sox as the Rays won the franchise's first pennant.
Red Sox: Fred Lynn
Debut: Sept. 5, 1974 vs. Brewers
Future Hall of Famer Jim Rice had already come up in mid-August 1974; in early September, the Red Sox called up the other half of what would become known as the Gold Dust Twins. Lynn created excitement right out of the gate, hitting .419 with two doubles, two triples and two homers to go with a 1.188 OPS in 43 at-bats as a September callup. He carried that momentum into 1975, when he was both the AL Rookie of the Year and AL MVP and helped Boston reach the World Series.
Yankees: Yogi Berra
Debut: Sept. 22, 1946 vs. Athletics
A 21-year-old Berra joined the Yankees for seven games at the end of the 1946 season, going 2-for-4 with a home run and two RBIs in his debut in the first game of a doubleheader victory at Yankee Stadium. Berra played 83 games the next year, winning the first of his record 10 World Series rings as a player. A three-time AL MVP, who earned 18 All-Star selections, Berra hit 358 homers with a lifetime .285 batting average and was elected to the Hall of Fame in 1972.
Guardians: José Ramírez
Debut: Sept. 1, 2013 vs. Tigers
The man known as J-Ram didn't make too much of an impact through his first three years in the big leagues, carrying a .239 average and a .644 OPS through 180 games. But beginning in 2016, Ramírez embarked on a stretch as one of the best players in the game. He registered an .891 OPS with 184 homers and 154 stolen bases over the next seven seasons, five of which saw the Guardians make the playoffs. Ramírez won a trio of Silver Sluggers and finished among the top three in AL MVP voting three times during this span. J-Ram became an All-Star for the fifth time in 2023.
Royals: Carlos Beltran
Debut: Sept. 14, 1998 vs. Athletics
Beltran got a hit in his first MLB game and went on to hit .276 that first September, impressive enough to earn him Kansas City's starting job in center field in 1999. That's when he showed the world his talent, earning the AL Rookie of the Year Award while hitting .293 with 22 home runs and 108 RBIs. Beltran hit 123 homers in a Royals uniform, and he went on to amass 435 career big flies, 2,725 hits and three Gold Gloves over his 20-year MLB career.
Tigers: Alan Trammell
Debut: Sept. 9, 1977 vs. Red Sox
The Tigers promoted a 19-year-old Trammell from Double-A Montgomery one year after they drafted him out of high school in 1976. Detroit put Trammell at shortstop on a Friday night at Tiger Stadium and paired him with his second baseman at Montgomery, Lou Whitaker, who made his debut the same night. The two ended up playing 1,918 games together, the most by a double-play duo in Major League history.
Twins: Tony Oliva
Debut: Sept. 9, 1962 vs. Tigers
Oliva, one of the best pure hitters of his generation, was a September callup twice before becoming a regular in 1964. The Cuba native played in nine games in September 1962, hitting .444 in nine at-bats. It was more of the same in '63, when Oliva hit .429 in seven at-bats. When he finally received consistent playing time in 1964, he won the first of back-to-back batting titles, hitting .323 en route to AL Rookie of the Year honors. Oliva went on to be an eight-time All-Star and a three-time batting champion in a 15-year career with the Twins. Knee injuries derailed his career, and he was regarded as one of the best players not in the Hall of Fame before finally being inducted in 2022.
White Sox: Luke Appling
Debut: Sept. 10, 1930 vs. Red Sox
By the numbers, the best September callup for the White Sox was Craig Wilson, who hit an astounding .468 with three home runs and 10 RBIs over 47 at-bats in 1998. (And Magglio Ordonez got called up on Aug. 29, 1997, missing the cutoff by three days.) But it was Appling who meant the most to the franchise, hitting .308 in 26 September at-bats during the 1930 season at the start of his Hall of Fame career with the White Sox.
Angels: Francisco Rodriguez
Debut: Sept. 18, 2002 vs. Athletics
A slew of injuries to their bullpen led the Angels to call up the 20-year-old Rodriguez, who struck out 13 over 5 2/3 scoreless innings in five September appearances to earn a spot on the club's postseason roster. K-Rod continued to electrify during the postseason, going 5-1 with a 1.93 ERA to help the Halos secure their first and only World Series title.
Astros: Joe Morgan
Debut: Sept. 21, 1963 vs. Phillies
The future Hall of Famer and two-time NL MVP with the Reds made his debut two days past his 20th birthday for the Colt .45s in 1963. Morgan appeared in only 18 games the next two seasons before his breakout rookie season in 1965 -- the year the Astrodome opened and the Colt .45s became the Astros. He played 10 years with Houston, hitting. 261 with 972 hits, 597 runs and 219 stolen bases.
Athletics: Jose Canseco
Debut: Sept. 2, 1985 vs. Orioles
Canseco burst onto the scene as a 21-year-old and quickly captured attention, batting .302 with five homers and 13 RBIs in 29 games for the A's, who would promote first-rounder Mark McGwire less than a year later. Canseco swatted 33 homers to earn the AL Rookie of the Year Award in 1986, and McGwire took home the honor the following season. The Bash Brothers combined for 200 home runs from 1988-90, each season ending with an AL pennant for Oakland.
Mariners: Edgar Martinez
Debut: Sept. 12, 1987 vs. White Sox
Martinez didn't make his MLB debut until his late-season callup as a 24-year-old third baseman. And even then, despite putting up a .372/.413/.581 slash line in 13 games, Seattle didn't give him a full-time shot until three years later. Once established, Martinez wound up becoming one of the greatest Mariners of all time, batting .312/.418/.515 over 18 seasons while earning seven All-Star berths, two AL batting titles and finally induction into Cooperstown.
Rangers: Juan Gonzalez
Debut: Sept. 1, 1989 vs. Royals
Gonzalez was twice a September callup. The first time was in 1989 after spending a year in Double-A, and he hit just .150 with one home run and seven RBIs. Gonzalez spent the 1990 season at Triple-A and was much better prepared by the time he was called up on Aug. 31. Gonzalez hit .289 with four homers, 12 RBIs and a .522 slugging percentage in 25 games. A two-time MVP with the Rangers, Gonzalez was one of the greatest players in franchise history.
NATIONAL LEAGUE EAST
Braves: Chipper Jones
Debut: Sept. 11, 1993 vs. Padres
Jones' Hall of Fame career began when he debuted as a ninth-inning defensive replacement for shortstop Jeff Blauser during a 13-1 Atlanta win over San Diego at Jack Murphy Stadium. Selected with the first pick in the 1990 MLB Draft, Jones went 2-for-3 with a double as he appeared in eight games (four as a pinch-runner and three as a pinch-hitter) for the '93 Braves, who won 22 of their final 29 games to complete a 104-win season and win the NL West by one game over the Giants.
Marlins: Josh Beckett
Debut: Sept. 4, 2001 vs. Cubs
The second overall pick in the 1999 MLB Draft, Beckett's much-anticipated debut came at age 21 in Miami. He threw six scoreless innings, allowing one hit while striking out five against the Cubs. Two years later, Beckett was the ace of the staff, and he threw a complete-game shutout in Game 5 of the NLCS vs. Chicago. In Game 7, he pitched in relief to help close out that series. Beckett was the 2003 World Series MVP. He spent the first five years of his 14-year MLB career with the Marlins.
Mets: Mookie Wilson
Debut: Sept. 2, 1980 vs. Dodgers
A college standout at South Carolina and a second-round Draft pick in 1977, Wilson stole 160 bases over four Minor League seasons to earn a callup when rosters expanded in '80. Immediately inserted into the leadoff spot for an out-of-contention Mets team, he hit .268 with seven steals. Mookie went on to compile 20.8 WAR over 10 seasons with New York -- tops all-time among Mets who debuted in September -- doing some of his most memorable work in the '86 World Series.
Nationals: Ryan Zimmerman
Debut: Sept. 1, 2005 vs. Braves
The Nationals made Zimmerman the first Draft pick in team history when they selected him fourth overall in 2005, and he reached the Majors that September. Zimmerman closed out the year with a slash line of .397/.419/.569 in 20 games, and he never left the Nats, wrapping up his 16-season career in D.C. in 2021.
Phillies: Jimmy Rollins
Debut: Sept. 17, 2000 vs. Marlins
The greatest shortstop in Phillies history made his big league debut a memorable one, tripling into the right-field corner at Veterans Stadium against Marlins right-hander Chuck Smith for his first career hit. "Watch him fly!" Phils broadcaster Harry Kalas said as Rollins raced around the bases. It was the first of Rollins' 2,455 career hits and 115 career triples.
Brewers: Jim Gantner
Debut: Sept. 3, 1976 vs. Tigers
It was four more years before the Wisconsin-born Gantner became a fixture at second base for the Brewers, part of a triumvirate with Hall of Famers Robin Yount and Paul Molitor that stayed together through 1992, when Molitor left via free agency and Gantner retired. Gantner was good but not great, beloved for his solid defense, hard-nosed play and longevity with his home-state team. His No. 17 is not formally retired, but it has not been given to another player since.
Cardinals: Stan Musial
Debut: Sept. 17, 1941 vs. Braves
The slugger whose name would become synonymous with St. Louis baseball debuted in the back end of a mid-September doubleheader, at age 20. Musial played in 12 games that month and hit .426/.449/.574 with a 1.023 OPS (and only one strikeout) in 49 plate appearances. Musial would make the team the following spring, at which point his Hall of Fame career began in earnest.
Cubs: Greg Maddux
Debut: Sept. 2, 1986 vs. Astros
Maddux was a second-round pick in 1984 and appeared in 14 games with the Cubs' Rookie-level Pikeville team that year, going 6-2 with a 2.63 ERA. He spent the '85 season at Class A Peoria, and he made 26 Minor League starts at Double-A and Triple-A in '86 before he was called up. The right-hander notched a win in his first big league start on Sept. 7 against the Reds, and Maddux would eventually pitch 23 seasons, including 10 with the Cubs. The Hall of Famer won four NL Cy Young Awards -- one with the Cubs, three with the Braves.
Pirates: Willie Stargell
Debut: Sept. 16, 1962 vs. Giants
Stargell played only 10 games in September 1962, and his rookie campaign in '63 wasn't particularly memorable. But "Pops" went on to enjoy one of the most distinguished careers in Pirates history. The Hall of Fame slugger brought home two World Series rings, an NL MVP Award, seven All-Star nods and a World Series MVP trophy. Stargell launched 475 career homers and drove in 1,540 RBIs while playing 2,360 games over 21 years -- all of them in a Pittsburgh uniform.
Reds: Joey Votto
Debut: Sept. 4, 2007 vs. Mets
Although Billy Hamilton's September 2013 callup in a pennant race might have been more electric at the time, Votto's debut six years earlier laid the foundation for his career as a franchise player. He batted .321/.360/.548 in 24 games and went on to finish second in the NL Rookie of the Year voting in '08. In 2010, Votto won the NL MVP as he evolved into one of the game's best and most consistent hitters.
D-backs: Miguel Montero
Debut: Sept. 6, 2006 at Marlins
Montero made his debut on the night Aníbal Sánchez threw a no-hitter against the D-backs. He then split time with Chris Snyder over the next couple of seasons before taking over as the starter in 2009. Montero compiled a 121 OPS+ in '11 when Arizona won the NL West, and he came back the next year and posted a 123 mark. He was recently named catcher on the team's 20th Anniversary team.
Dodgers: Fernando Valenzuela
Debut: Sept. 15, 1980 vs. Braves
Purchased from his Mexican League team for $120,000 the previous year, Valenzuela was a starter the entire 1980 Double-A season at San Antonio, and he was promoted to the Dodgers as an extra bullpen arm for the stretch run. He tipped off his future stardom with 17 2/3 scoreless innings in 10 appearances as the Dodgers finished in a first-place tie with the Astros, losing a one-game NL West tiebreaker game when manager Tom Lasorda started Dave Goltz instead of Valenzuela. The following year, Valenzuela was the first to win the Cy Young and the Rookie of the Year in the same year, and the Dodgers won the World Series.
Giants: Buster Posey
Debut: Sept. 11, 2009 vs. Dodgers
Posey was selected as the No. 5 pick in the 2008 Draft. Every team hopes to land a foundational player when picking that early in the Draft, and Posey absolutely was that for the Giants. Following a seven-game introduction in 2009, Posey guided the Giants in 2010 to their first championship in San Francisco while taking home Rookie of the Year honors. Two years later, as the Giants won another World Series, Posey captured the NL batting title and was feted as the NL MVP. A third World Series title came in 2014. By the time he retired in 2021, Posey had racked up seven All-Star selections and five Silver Sluggers. His 129 career OPS+ is tied for second highest among players to catch at least 1,000 games.
Padres: Khalil Greene
Debut: Sept. 3, 2003 vs. D-backs
Greene was OK in September 2003, posting a .671 OPS in 20 games. But his callup paved the way for a 2004 season in which he finished second to Jason Bay in NL Rookie of the Year voting. Greene then helped the Padres to consecutive division titles in '05 and '06, and a near-miss in '07.
Rockies: Ubaldo Jiménez
Debut: Sept. 22, 2006 vs. Rays
The Rockies generally have their top prospects get their feet wet before the fool's gold days of September. Of the top 12 players in club history in WAR, Jiménez and German Márquez are the only ones who debuted with Colorado in September. In 2010, Jimenez threw a no-hitter, went 19-8 and set what was then a club record with a 2.88 ERA.