The warning is simple: Never believe what you see in Spring Training or September.
Those are words baseball people live by, and rightfully so. Small sample sizes -- especially during those times -- can be deceiving.
But that doesn't mean they can't affect a pennant race or a team's views of its future.
Major League Baseball rosters expand to 40 on Sunday. Contenders will add pitching depth and specialty players, while those out of the race will look ahead to next season and hold 29-day auditions.
Historically, sometimes September numbers are a sign of things to come.
The pinnacle of September callup success stories was then-Angels reliever Francisco Rodriguez, who eventually earned a postseason roster spot as a 20-year-old thanks to a teammate's injury. The rest is October lore, as "K-Rod" was a dominant late-inning presence for the Angels en route to their 2002 World Series championship.
The first act of 1981's "Fernandomania" came in September 1980, when Dodgers lefty Fernando Valenzuela allowed zero earned runs and struck out 16 in 17 2/3 relief innings.
Other future All-Stars have shown of things to come in September:
• Josh Beckett started four games in late 2001 for the Marlins, posting four quality starts with a 1.50 ERA.
• Marlins right-hander Josh Johnson was called up from Double-A Carolina in 2005. He pitched well in relief, and also once filled the rotation spot of A.J. Burnett, who was dismissed from the team for making disparaging comments about the club.
• Reds first baseman Joey Votto had a .321/.360/.548 slash line in 24 games in 2007, a year before he finished second in the National League Rookie of the Year Award voting.
It remains to be seen if this year's callup crop will mature into players of that caliber. At least for contenders, the biggest thing they're looking for is niche players with speed, as well as extra bullpen arms.
That's what the Cardinals will do by promoting speedy outfielder Adron Chambers and four to six relievers to help with an overworked bullpen.
The Red Sox will look to use recently-acquired Quintin Berry as a pinch-runner for them in September and beyond. Jackie Bradley Jr. -- who started the season as Boston's left fielder and has played solidly at Triple-A -- could help in that department as well.
And as Red Sox Nation knows from Dave Roberts and the 2004 playoffs, speed certainly can impact a postseason run.
Speed can't be mentioned without Reds No. 1 prospect Billy Hamilton, who is not currently on the club's 40-man roster. Hamilton set professional baseball's single-season record with 155 steals between Class A Advanced Bakersfield and Double-A Pensacola last season, and has followed that with 73 steals in 88 attempts at Triple-A Louisville this season.
"It's taken him a little time to adjust to some of the pickoffs and some of the baserunning stuff that you need to learn to win. That's what I've been told," Reds manager Dusty Baker said. "Speed is great. Speed kills, but speed also kills you if it's out of control."
As for non-contenders, here are a few late-season callups to keep an eye on:
• Mariners right-hander Taijuan Walker -- MLB.com's No. 4 prospect -- will make his Major League debut Friday against the Astros.
• The Mets' Matt den Dekker isn't a heralded prospect, but he is an elite defender who could play a significant role with his power and speed.
• Marlins left-hander Brian Flynn, who stands 6-foot-7, could wind up as the sleeper acquisition in the 2012 trade with the Tigers that also sent Anibal Sanchez and Omar Infante to Detroit for Jacob Turner and Rob Brantly.
• San Diego outfielder Reymond Fuentes and left-hander Robbie Erlin figure to get auditions with the Padres this month; though the Friars are pretty set in the outfield. Erlin started Wednesday, throwing six innings of one-run ball to earn the win.
• Astros center fielder George Springer has combined for 37 home runs and 43 stolen bases at Double-A and Triple-A this season, though general manager Jeff Luhnow said last week he would remain with Oklahoma City through its playoff run. It remains to be seen if Springer will come to Houston when his Minor League season is over.
Cash Kruth is a reporter for MLB.com. Follow him on Twitter at @cashkruth.