Mike Trout has a torn meniscus in his left knee.

From mlb.com:

Trout said the hardest part is he’s not even sure when he sustained the injury. He had no knee issues leading up to Monday’s series opener, but felt something while jogging off the field in the third inning. Trout continued to play through the injury, including stealing second base in the seventh inning and scoring the eventual winning run from second base on a wild pitch. But his knee continued to worsen and he underwent an MRI exam on Tuesday that revealed the damage.

“It’s crazy because when I look back, I don’t even know when I did it,” Trout said. “In the third inning when I was going back to the dugout, I felt an ache, but just like a little ache where it was more like, ‘This is weird.’ Didn’t think anything of it and was hitting and running and didn’t feel anything. Stole second, felt nothing. It was just after I did activity and sat back down and got back up, that’s when I felt it.”