SAN FRANCISCO — Giants' left-handed starter Randy Johnson left today's 7-1 loss to the Houston Astros with a left shoulder strain after committing a throwing error in the top of the fourth inning.

"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."

Johnson, who has 303 career victories, gave up solo home runs to Hunter Pence and Jason Michaels, and then threw the ball down the right field line on Oswalt's soft grounder back to the mound.

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.