MLB testing new Atlantic League rules, including designated runner

Major League Baseball is testing out a trio of rule changes in the Atlantic League this year.

The independent league is introducing a designated pinch runner, limiting pitcher disengagements to one per at-bat, and bringing back its "double hook" rule that promotes longer outings for the starting pitcher.

Each team will be able to designate a runner who is not otherwise in the starting lineup prior to a game. That player can then be substituted in for any baserunner at any time without officially removing the baserunner from the game.

MLB limited disengagements by pitchers this year as part of an effort to create more stolen bases and speed up the pace of play. Pitchers in the majors are currently limited to two disengagements per plate appearance.

The "double hook" rule, meanwhile, will be used again after being introduced in the Atlantic League last season. It dictates that if a starting pitcher is removed from a game before logging five innings pitched, the same team loses its designated hitter for the remainder of the contest. This could effectively end the strategic use of the opener - a reliever who starts the first inning of a game instead of acting as a closer.

The Atlantic League is unaffiliated with MLB but signed a deal in 2019 to test out potential rule changes as a partner league. Its season begins April 28.