FOXBOROUGH, Mass. (AP) -- Patriots quarterback Tom Brady limped off the practice field after appearing to hurt his left leg while throwing a pass.
Brady had left tackle Nate Solder pushed into him by a defender after throwing a fade to rookie Aaron Dobson that fell incomplete, and the former league MVP ended up on the ground holding his leg.
He took a few more snaps before talking with coach Bill Belichick and walking off gingerly but without assistance.
He was met by a member of the team's athletic training staff and walked to the adjacent indoor practice facility. He had not returned to the field after about 30 minutes.
About an hour later, team president Jonathan Kraft told 98.5 The Sports Hub, "I don't believe it's serious but I do not know."
Brady tore the anterior cruciate ligament in his left knee during a game in 2008 and missed most of that season.
The Patriots were holding a joint practice with the Tampa Bay Buccaneers. Ryan Mallett replaced Brady at quarterback.
Brady's injury -- severity unknown -- comes at a time when the NFL is experiencing a spike in ACL injuries.
In the first two-plus weeks of training camp, there have been at least 15 season-ending knee injuries. Even Atlanta Falcons President Rich McKay, the chairman of the NFL's Competition Committee, told USA TODAY Sports, he thinks "there have been more ACL injuries than ever."
According to Edgeworth Economics, which recently conducted an independent analysis of the data provided by the NFL Players Association, ACL injuries are on the rise, although the research doesn't differentiate between a sprain and a tear.
For instance, in the 2004 preseason, there were 13 ACL injuries. That number jumped to 24 in last year's preseason, and it appears the 2013 preseason could surpass that.
"The majority of ACL tears are non-contact tears," said orthopedist Neal ElAttrache from the Kerlan-Jobe Orthopedic Clinic in Los Angeles. "The non-contact ACL tear results when you're doing something that your brain hasn't planned or patterned, so you misstep. Or, unexpectedly, a blocker gets pushed into you with your foot planted."
ElAttrache repaired Brady's torn ACL and MCL in 2008.