According to Tom H's blog, Lorenzo Cain hurt himself trying to make a diving catch and on-site doctors think he tore his MCL.
If that's the case, he's looking at three or so months of rehab.
Read Tom H's post here
As someone who tore the same MCL twice, I can tell you it's no picnic. The MCL is on the inside of the knee and really affects your ability to plant and cut. When I tore it the second time, it officially ended my not-so-illustrious soccer career as I decided the ability to walk for the rest of my life was more important.