Major League Baseball on Thursday announced a multitude of major changes to how business is conducted and how the game is played, including expanding rosters from 25 to 26 players and eliminating in-season trades after July 31.

The change with the apparent potential to affect the game the most on the field also might be the one that could make an endangered species out of some left-handed pitchers: requiring a reliever to face at least three batters (or otherwise finish the inning).

‘‘It was a little shocking, to be honest, that that’s one of the things they came up with,’’ Cubs left-hander Brian Duensing said. ‘‘I think it’ll take jobs away from some pitchers.

‘‘There’s a lot of lefties out there that their career is coming in and getting lefties out. . . . Some guys make their living by getting a hitter out. Now it might be a little different for them.’’

The three-batter minimum is among the rules to be implemented in 2020. Also among those on the 2020 list are the expanded roster and a limit of 28 players (down from 40) on September rosters.

Among the new rules that will apply immediately are a reduction of mound visits from six to five per game, a $1 million prize for the winner of the All-Star Home Run Derby, changes in All-Star fan voting and a single trade deadline of July 31, which means no more waiver deals in August or September.