Tom Brady -- WRITHING IN PAIN ... Injures Knee at Patriots Practice

Another HUGE blow for the New England Patriots (and female football fans worldwide) -- 'cause Tom Brady suffered a brutal knee injury during a team scrimmage today ... and it looks bad. Real bad.

Brady -- who was wearing a red "keep your damn hands off me" jersey at the time -- had dropped back to pass when the pocket collapsed on him.... and it appears a lineman fell on his knee.

Brady dropped like a sack of potatoes ... clutching his left knee -- the same knee he destroyed on opening day back in 2008.

Brady was eventually helped off the field and is currently undergoing evaluation.

Paging Tim Tebow ...

3:20 PM PT An MRI on Brady's knee was negative ... according to ESPN.