New England Patriots star quarterback Tom Brady left the practice field with an apparent injury to his left knee, the same knee that cost him his season five years ago with a torn anterior cruciate ligament.

A fan posted a video via Twitter that shows Brady go down and clutch his knee.

This injury may not be that severe, though.

Several reporters said Brady grabbed his knee after an incomplete throw without being hit, limping off the field before going back on the field to continue. Then, Brady jogged off and went to get checked out after talking with Patriots coach coach Bill Belichick, offensive coordinator Josh McDaniels and head trainer Jim Whalen.

It's unclear what the nature of this injury is, but obviously it's a concern for the Patriots.

If Brady is sidelined, their new starter would be Ryan Mallett and the backup would be Tim Tebow.

Stay tuned for further developments.

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Aaron Wilson covers the Ravens for The Baltimore Sun.