Other changes for this season further Commissioner Rob Manfred’s goal of speeding up the pace of play. The average nine-inning game took three hours last season, down by five minutes from 2017 but still 14 minutes longer than the average game in 2005.

Pace-of-play rules for 2019 will not include a pitch clock but will include a reduction in mound visits, from six to five, and in the time between innings. While the change remains to be reviewed with the holders of broadcast rights, those breaks would be reduced to two minutes for all regular-season games, from 2 minutes 5 seconds for local telecasts and 2:20 for national telecasts.

The more significant changes will begin in 2020. Rosters will increase by one slot, to 26 players, through the end of August. In September, though, the maximum roster size will be reduced to 28, from 40.

“September games are a little outrageous at times, especially when you get two teams that are really competing for a postseason spot,” Duffy said. “They want to give themselves the best chance to win a game, and if that means bringing in five or six relievers in one inning, they’ve got them down there, why not use them? I think you’re going to see a little more realistic games in September.”

He added, “And the reality is, to have 80 percent of your season be decided in a certain way and then all of a sudden in the most important games, everything flips completely — that’s a little weird, too.”

Teams will have a limit, still to be determined, on the number of pitchers allowed on the active roster — and all pitchers will have to pitch to a minimum of three batters, or to the end of a half-inning. Yankees reliever Zack Britton said players were conflicted on that rule, which M.L.B. will impose on its own with the union agreeing to not challenge it.

“It changes strategy, for sure,” Britton said. “I think nowadays there’s not as many specialized pitchers that are just righty or just lefty. But obviously, if there are guys out there, it limits their role and job opportunity.”