Boston Red Sox starter Richard Fitts left Saturday's 3-2 loss to the White Sox in Chicago with right shoulder pain.
The right-hander allowed just two singles, striking out five and walking one, in five innings before leaving as he faced Miguel Vargas leading off the sixth.
He'll return to Boston on Sunday and then will undergo an MRI, according to manager Alex Cora.
"Whenever you have to come out of the game, obviously you talk MRIs, you always get concerned," Cora told reporters after the game. "He really wanted to keep going.