Wake up, September has almost begun.

The arrival of the 2021 season’s last full month brings playoff clinches, nail-biting finishes in division and Wild Card races and final arguments for major award cases. But perhaps the most notable thing about Sept. 1 on the baseball calendar is roster expansion.

That said, this isn’t the roster expansion you might remember from past years. Here’s what fans need to know as Major League rosters grow in the month of September.

Roster limits
Under current rules, Major League teams are allowed to carry 26 players on their everyday active rosters. Clubs can add a 27th player on days in which they play a doubleheader. In September, those roster limits expand to 28 (29 on twin bill days). Teams must use every one of those 28 spots and can also carry a five-player taxi squad with them on road trips.

Prior to 2020, Major League rosters could expand to include as many members of the system’s 40-man roster as the front office wanted. While the extra help in the heat of a playoff race was certainly welcomed, bloated rosters often led to lengthy games and teams playing with very different roster priorities. Capping the roster at a still-expanded 28 can give Major League clubs that extra boost toward the end of the season while making sure matters don’t get too out of hand.

• 26-man roster rules

Who can be called up
This is still not quite a “normal” Major League season while the sport deals with the COVID-19 pandemic, and fans might recall that protocols in early 2021 allowed clubs to promote only Triple-A players directly to the Majors due to their Tier 1 status. As vaccination availability has increased since the spring, this portion of the protocol has been relaxed and amended. Now, players from all levels are allowed to be added to a Major League roster so long as they are vaccinated. If a player is not vaccinated, then they still must be in Triple-A prior to their arrival in the Majors.

New Minor League schedule
One reason why rosters could expand to 30-plus players in the past was that the Minor League regular season traditionally ended on or around Labor Day. Because the Minor League season was pushed back to May this year, that is not the case, with schedules extending through September. Triple-A clubs, in particular, will participate in a postseason tournament called the Triple-A Final Stretch that concludes Oct. 3, the same day the Major League regular season will also end.

In essence, players won’t be sent to the Majors in September just to get work in. They can still do that in the Minors. If they do get the call, it should be to make meaningful contributions down the stretch.

Rookie eligibility
Before 2020, days spent on a Major League roster did not count toward rookie eligibility if they came in September. In theory, a player could have 44 days on Aug. 31 and still not graduate from rookie status even if he was in the Majors for the duration of September.

That changed in 2020, and the alteration continues to stick in '21. Days spent on the active Major League roster will count toward the 45 needed to graduate from rookie status. So prepare for more prospect graduations in the weeks ahead.

Top 50 prospects who could get the call

Bobby Witt Jr., SS, Royals (No. 3): There have been rumblings since the spring that Witt would make his Major League debut at some point in 2021, and he’s only helped his case with a 20-20 season at Double-A Northwest Arkansas and Triple-A Omaha. If Kansas City is serious about making Witt the cornerstone of the franchise, it might want to give him a look at The Show now rather than waiting several more months.

Vidal Bruján, OF/2B, Rays (No. 22): No one might be better built for September roster expansion of any size than Bruján. His 70-grade speed plays well off the bench. He has experience at second, third, short and all three outfield spots. He can switch-hit. Bruján’s first go in the Majors didn’t go to plan (2-for-26 in 10 games), but you can bet Kevin Cash and his staff could find many uses for him in their attempts to sew up the American League East.

Cristian Pache, OF, Braves (No. 40): It seems like a long time ago, but it’s important to recall Pache was a starting outfielder for the Braves in last year’s postseason. Many thought he would have graduated by now, but a tough go with the bat has kept him at Triple-A Gwinnett for much of the summer. Good news: Pache has enjoyed his best offensive month in August, and that could help get him another look with the National League East leaders. His defense will still certainly play on the Major League grass.

Check out Jonathan Mayo's recent piece for more on top prospects that could specifically help contenders this September.