Section 4: Players & Officials
4.1 Teams and Rosters (OBR Rule 4.01–4.03)
Per OBR Rule 4.01, a regulation game is played between two teams of nine players each. Each team's manager must deliver to the umpire-in-chief and the opposing manager a batting order containing each starting player's name and position at least 5 minutes before the scheduled game time for day games and 10 minutes for night games. Once a player's name is submitted in the batting order, they are considered in the game for the purposes of lineup integrity.
MLB rosters during the regular season consist of a 26-player active roster (expanded to 28 from September 1 through the end of the regular season) and a 40-man roster. Substitutions must be reported to an umpire, who shall announce or cause to be announced the substitution to the official scorer and the public address announcer (OBR Rule 5.10(b)).
4.2 Substitutions (OBR Rule 5.10)
Per OBR Rule 5.10(a), the manager shall immediately notify the umpire-in-chief when a substitution is made. A substitute player, once entered in the game, cannot be removed and re-entered; a player removed from the game is finished for that game. The designated hitter (DH), introduced in the American League in 1973 and adopted universally in MLB starting with the 2022 season, is covered under OBR Rule 5.11.
4.3 The Designated Hitter (OBR Rule 5.11)
Per OBR Rule 5.11, a hitter may be designated for any pitcher, but not be a fielder. Selection of a designated hitter is optional and must be announced before the game. If the DH is removed for a pinch hitter or pinch runner, that player becomes the new DH. If the DH takes a fielding position, the team loses the DH for the rest of the game and the pitcher must bat in the order vacated by the player replaced by the pitcher.
4.4 The Umpires (OBR Rule 8.01–8.04)
Per OBR Rule 8.01, the umpire-in-chief (home plate umpire) shall have general authority to enforce all rules, call balls and strikes, make all rulings on the batter, and have authority to remove players, managers, or coaches for violations. A full crew for MLB games consists of four umpires (home plate, first base, second base, third base). In postseason play, six umpires are used (adding left field and right field umpires).
- Rule 8.02(a): Umpires shall not reverse a decision unless they determine that the rule was misapplied.
- Replay Review: Under MLB's Replay Review system (in effect since 2014, expanded in subsequent seasons), managers may challenge reviewable calls. Each team receives one challenge per game; if the challenge is successful, the team retains the challenge. Beginning in 2020, the umpire crew chief may also initiate a review on certain calls. Non-reviewable calls include balls and strikes, check swings, and interference/obstruction in certain circumstances.
- Rule 8.04: Umpires are authorized to remove any player, manager, or coach who uses objectionable language or acts in a manner deemed to be against the best interests of the game.
4.5 The Manager and Coaches (OBR Rule 4.05–4.06)
Per OBR Rule 4.05, the manager is responsible for the conduct of the team and must be in uniform. Each team is entitled to a manager and up to six coaches (as per current MLB rules). Coaches must remain in the coaches' boxes at all times while a game is in progress, unless coaching a runner or receiving permission from the umpire to confer with a player.