For National Association play only, the starting pitcher or any
substitute pitcher is required to pitch to a minimum of three
consecutive batters, including the batter then at bat (or any sub-
stitute batter), until such batters are put out or reach first base,
or until the offensive team is put out, unless the starting pitcher
or substitute pitcher sustains injury or illness which, in the
umpire-in-chief’s judgment, incapacitates him from further
play as a pitcher.
For National Association play only, the starting pitcher or any substitute pitcher is required to pitch to a minimum of three consecutive batters, including the batter then at bat (or any sub- stitute batter), until such batters are put out or reach first base, or until the offensive team is put out, unless the starting pitcher or substitute pitcher sustains injury or illness which, in the umpire-in-chief’s judgment, incapacitates him from further play as a pitcher.
https://content.mlb.com/documents/2/2/4/305750224/2019_Offic...