During a practice against the Tampa Bay Buccaneers in Foxborough, Mass., Brady was throwing a pass when he fell to the ground without being hit. He grabbed his left leg.
He left the field gingerly but on his own, then reportedly made a brief return to the field before leaving again to meet with the training staff.
The franchise quarterback then talked with Coach Bill Bellichick and offensive coordinator Jeff McDaniels before going to the team's indoor training facility.
Brady is backed up by Ryan Mallett and Tim Tebow.
Back in 2008, Brady tore the anterior cruciate ligament in his left knee during the opening game and missed the rest of the season. The team went 11-5 behind then-backup Matt Casell and barely missed a playoff berth.