Gagne, who has 45 saves in 47 chances, allowed a run on two hits in the 4-2 win.
Tracy said Gagne felt stiffness in his right shoulder when he played catch Tuesday and had an MRI, which revealed no structural damage. Gagne was diagnosed with tendinitis and bursitis Wednesday and had a cortisone injection.
"I think he will be available, in all likelihood, [Thursday] night," Tracy had said. "This won't govern how we use him in the future. What will govern that is how he feels day to day."
Asked how Gagne was injured, trainer Stan Johnston told the Los Angeles Times, "Overuse, or it might have been warming up too quick."
"He will not pitch until the stiffness goes away. [The MRI results] are really good news. A lot of players have this. The important thing is to take care of it in a hurry."
Gagne told the newspaper that he thought the stiffness could have occurred from "sleeping on [the shoulder] the wrong way." He also told the Times he had experienced stiffness before in the elbow of his pitching arm, but never in the shoulder.
"It's like a dead arm," Gagne said.