In an effort to reduce the number of pitching changes and, in turn, cut down the average time per game, MLB will institute a rule change beginning in 2020 that requires pitchers to either face a minimum of three batters in an appearance or pitch to the end of a half-inning, with exceptions for injuries and illnesses.
Currently, Rule 5.10(f) states that the starting pitcher must pitch to one batter until that batter is put out or reaches base, and Rule 5.10(g) states that any reliever must pitch to one batter until that batter is put out or reaches base, or the offensive team is put out, with exceptions for injuries and illnesses. These rules will be in effect through the end of the 2019 season.
The 2020 change will likely cause a decline in the number of specialists -- such as the "LOOGY" (lefty one-out guy) -- who are primarily brought in for certain matchups.
Watch: Dan Jennings starts a game and is removed immediately after retiring leadoff hitter Matt Carpenter in the first inning. Barring injury, this will no longer be allowed beginning in 2020.