Originally Posted by Nate From Ohio
There are 25 roster spots on an MLB roster. If one guy gets hurt he gets put on the Disabled List (DL) thus there is an open roster spot, so the MLB team will usually bring up a guy from the minor leagues to fill that roster spot which was vacated by the guy goin gon the DL. When the guy on the DL is ready to play again a roster spot must be freed up for him, thus someone either has to be sent to the minors, or put on the DL.
forgot to mention if a player is really really bad