Yes. Every player added to an active roster in September must already be on his club’s 40-man roster. There is no exception, and it is the single biggest constraint on who actually gets promoted.
This catches people out because the best prospect in a farm system is often not on the 40-man at all.
The Two Rosters
Baseball runs two rosters at once. The active roster is who can play today — 26 players for most of the season, 28 from September 1. The 40-man roster is the pool of players under major league contract, whether they are in the majors, the minors or on the 10-day injured list.
Promotion means moving from the 40-man to the active roster. A player not on the 40-man cannot simply be added; the club must first select his contract, which requires an open 40-man spot.
Our explainers on the 40-man roster and the difference between the two rosters cover the structure in full.
What Happens When the 40 Is Full
Most contending clubs carry a full 40-man roster by late August. To promote anyone, they must clear a spot first, and there are only a few ways to do it:
| Move | What it costs |
|---|---|
| Designate for assignment | The player can be claimed by another club and lost outright |
| Move to the 60-day IL | Free, but only available if someone is genuinely hurt long-term |
| Outright a player off the roster | Requires waivers; veterans can refuse and become free agents |
| Release | The club still owes the remaining salary |
Each of those costs the organisation something real. That is why the two expansion spots often go to players already shuttling between Triple-A and the majors — they are on the 40-man already, so promoting them costs nothing.
Why the Best Prospect Often Stays Down
Adding a prospect to the 40-man starts his option clock and exposes him to the Rule 5 draft calculus. Clubs protect prospects on the 40-man only when they have to — typically in the November before a player becomes Rule 5 eligible.
So a top-100 prospect tearing up Triple-A in August may simply not be on the 40. Promoting him means burning a roster spot the club had planned to use elsewhere, and starting a service-time clock earlier than intended. Our service time explainer covers why that matters financially.
The Old System Was Different
Before 2020, September rosters expanded to the full 40. Every player on the 40-man could be activated, so the question of who was on it mattered far less — if you were on the 40, you were eligible to play.
Now only two extra spots exist, so being on the 40-man is necessary but nowhere near sufficient. A club with 40 players under contract still has to choose just two.
The Bottom Line
Yes, September call-ups must be on the 40-man roster. If the 40 is full, a club has to designate, outright, release or injure-list somebody before promoting anyone. That friction, more than talent, decides most September promotions — and it is why the best name in Triple-A frequently stays there.