Gallo was unable to bat in the ninth inning after being hit by a Tim Mayza pitch in the seventh.
"He's getting X-rays on everything, and we'll see what we have when we get that," manager Aaron Boone said. "He couldn't hit, and we took him out of the game, so we'll see what we have now that we'll have X-rays, and we'll see what happens overnight."
Boone also said Luke Voit was getting treatment after tweaking his knee but expects him to be OK. Voit appeared to hurt himself running to first after striking out later in the inning.
Voit had been dealing with a left knee bone bruise.
"On his knee, that bugs him sometimes, he just got slammed on it," Boone said. "Hopefully, he should be OK. He gets that sometimes. Tonight, when he stopped, he got bit on it a bit. He got some treatment, and we'll see what we have tomorrow. I'm hoping he's OK."
Boston, which beat Baltimore, closed within one game of the wild-card-leading Yankees, and Toronto is one game back of the Red Sox. Seattle, which began the day 2½ games behind the Yankees, hosted Oakland in a late game.
Information from ESPN's Marly Rivera was used in this report.