FOXBOROUGH, Mass. -- Patriots linebacker Brandon Spikes underwent an MRI on his injured knee Monday and it revealed an medial collateral ligament strain, a league source told ESPN NFL Insider Adam Schefter. Spikes is week to week with the injury.

The Patriots have a key AFC East battle at the Jets Sunday night.

Spikes injured the knee defending a third-down pass near the front of the end zone early in the third quarter of Sunday's loss to the Giants. He was examined on the sidelined before heading to the locker room. He did not return to the game.

Patriots coach Bill Belichick confirmed the tests on Monday, but he didn't offer a diagnosis.

"He's having some tests done today, we'll see how he responds," said Belichick. "It's hard to tell right after the game, a lot of adrenaline going and all that. We'll take a look today, see what it looks like tomorrow. Sometimes those things clear up, they're bruises. Sometimes they are more than that and it takes a little bit longer. We'll have a little better idea tomorrow and we'll see whether he can practice on Wednesday."

Spikes' 2011 season started slow due to injuries (he was inactive Week 1), but his snaps have ramped up ever since as he's settled into a starter's role at middle linebacker. Spikes has 44 tackles through seven games.

Chris Forsberg covers the Patriots and Celtics for ESPNBoston.com.