This article is part of our Farm Futures series.
Predicting when players like Vladimir Guerrero Jr. or Yordan Alvarez will be called up to make their big-league debut during the first five months of the season is an abstract activity that requires instincts, experience and a good feel for how a player's skills will play at the highest level. However, predicting which prospects and former prospects will be recalled when the calendar turns to September and rosters expand is more science than art.
In 90 percent of cases, a player's standing on the 40-man roster will open or shut the door on any conversation about a September callup. Very rarely does it make sense to add a player to the 40-man roster after Sept. 1. This is especially true of teams that are not contending.
There are three instances when a team will add a prospect to the 40-man roster in September. The first instance is when a contending team has a need and the only internal replacement is a prospect who is not on the 40-man roster. This is pretty rare, but it happened last year with Kyle Wright fortifying the Braves' bullpen and a couple years ago with Alex Verdugo and Walker Buehler joining the Dodgers for the final month. The second scenario is when a team sees value in getting a look at a player against big-league pitching over the final month, even if they expect that player to head back to the minors at the start of the next season. For instance, Willie Calhoun was