"I'm going to make this short," Johnson said. "Right now it's a strain and I'm going for an MRI tomorrow. That's all I'm going to say."
Johnson originally hurt the shoulder when took an awkward swing against Roy Oswalt in striking out in the bottom of the third. He grabbed his shoulder as he walked back to the dugout.
"If we think there's anything going on we'll give him extra time," Giants manager Bruce Bochy said. "He hurt it on the swing but said he felt fine until the throw."
Trainers raced to the mound after the play, examined Johnson and made the decision to end his stint. Justin Miller took over with two outs in the fourth.
"He was throwing slower than he normally does," Giants' catcher Bengie Molina said. "He usually throws in the 90s and all of a sudden he was throwing 81-82. Let's hope he's OK."