|When an umpire leaves, the questions begin.|
Answer: Subject to Crew Chief discretion, the second base umpire shall replace an injured plate umpire (MLB Umpire Manual VI). If the injury occurs before the game is official, this umpire's next game assignment will be at third base. If the injury occurs after the game is official, this umpire's next assignment will be at first base.
|The injury's extent dictates response protocol.|
|Some injuries are clearly less severe.|
Looking through the game-ending injury log for plate umpires in the majors over the course of the 2016 season, we see that of the 14 instances in which a plate umpire has left during the pendency of a game, he has been replaced:
> 6 times by the 2B Umpire (43%).
> 5 times by the 1B Umpire (36%).
> 3 times by the 3B Umpire (21%).
The answer seems somewhat consistent with the MLBUM guideline, as second base umpires have gone behind the plate more often than any one of the other positions have, but it happened that way in less than half of the injury cases in 2016. As the U2-to-UIC guideline is subject to Crew Chief discretion, we can observe this protocol by crew.
|Other injuries require more extensive care.|
On the other hand, Dale Scott's crew has seen two such injuries (6/19 Scott; 7/16 Scott), resulting in the first base umpire filling in behind home plate on both occasions.
Some crew chiefs may opt to maintain the integrity of the rotational order by having the 1B Umpire move to home plate if the injury occurs early in the game (e.g., 5/26 Guccione [1st inning]; 7/16 Scott [1st]; 5/23 Nelson [3rd]; 7/15 Fairchild [1st]). Similarly, some may choose to preserve the alignment if the injury occurs extraordinarily late in the game by having the 3B Umpire work home plate for the final moments (e.g., 6/23 Emmel [9th, 2 outs; the game ended on the very next pitch]).
|Second base is left vacant in a three-person alignment.|
The second base umpire will also have a full day off between plate games, even if it is only one day: If either a first or third base umpire works the plate, that usually will be a consecutive plate day, unless the crew/that umpire has a day off immediately before or after the game.
|A call-up's first game will be on the bases.|
Finally, some base umpires—especially new-to-crew umps and during the first or last game of a series—might not even have their plate gear with them. See this article about travel delays and CB Bucknor wearing street clothes during an MLB game for more information.
In all, there is much to consider when assigning a replacement plate umpire during a live game, and although MLBUM suggests the second base umpire, there are a handful of extenuating circumstances that can have an impact on the Crew Chief's decision.