FOXBOROUGH, Mass. >> Tom Brady walked off the field under his own power. Several teammates didn’t think he was hurt badly when he grabbed his left knee after being knocked down.

Yet the possibility that the New England Patriots might lose their star quarterback caused a scare at the team’s joint practice with the Tampa Bay Buccaneers on Wednesday.

A person with knowledge of the injury said an MRI was negative and Brady was day-to-day with a left knee sprain. Asked if Brady might play Friday night in an exhibition game against the Buccaneers, the person said he didn’t know.

The person spoke on condition of anonymity because the Patriots didn’t make an announcement.

The two-time league MVP had a season-ending injury to the same knee in the 2008 opener.

Brady was injured midway through practice Wednesday when left tackle Nate Solder was pushed into him by Tampa Bay defensive end Adrian Clayborn. Brady, entering his 14th NFL season, fell, rocked backward on the ground and held his left knee.

He went to the sideline then returned for a few more plays before talking with coach Bill Belichick and leaving the field toward the team’s indoor practice facility next to it.

“You’ve always got to stay away from the quarterback (in practice),” Clayborn said. “You got a guy on his heels and my instinct is to keep going, so that’s what I did.”

The contact came just as Brady threw a long incompletion down the right sideline toward rookie Aaron Dobson.

“We’re always working to protect,” Solder said. “I’ll have to see what happened on the film. I screwed some things up here and there.”

In 2008, Brady tore the anterior cruciate ligament in his left knee. The Patriots missed the playoffs that season. He has played every game since then and the team reached the playoffs in all four seasons.

New England is favored to win the weak AFC East for the 10th time in 11 years. Losing Brady would have made that much tougher.

After Brady left the field, third-year pro Ryan Mallett played with the first offensive unit and Tim Tebow, on his 26th birthday, played with the second team. Mallett threw an interception soon after taking over.

“I’m just trying to do my job,” Mallett said. “I’m sure Tom will be fine and we’ll just go on.”

In his first series after Brady left, Tebow completed three of seven passes and looked fairly sharp. One of his passes was dropped.

Tebow left practice without speaking with reporters.

“Anytime anybody, especially a teammate, goes down it’s a dark cloud. It’s unfortunate,” Patriots running back Shane Vereen said. “He went back out there, but we’ll see.”

Running back LaGarrette Blount was optimistic.

“He walked off,” Blount said. “I’m sure he’s cool. I’m sure he’s fine.”

But shortly before Blount spoke, a black helicopter landed on a field adjacent to the one where Brady was injured. Cameramen, apparently concerned that Brady might be airlifted to a hospital, rushed to record the landing.

“The helicopter has nothing to do with Tom,” Patriots owner Jonathan Kraft said on WBZ-FM.

Tampa Bay coach Greg Schiano said he and Belichick stress to their players to rush the passer but let him throw the football and go past him.

“I didn’t get to see” the hit on Brady, Schiano said. “I saw him laying there afterward. … I hope he’s OK.”

Losing Brady would leave the Patriots without a proven starting quarterback. Mallett has thrown just four passes in his first two seasons and has been inconsistent during training camp. Tebow signed as a free agent on June 10 and has struggled with his throwing mechanics, a problem he’s had since being drafted by the Denver Broncos in the first round in 2010.

“The big thing is just his familiarity with the offense, the way we do things — the play calling, the reads, formationing, just general things we do,” Belichick said of Tebow. “The quarterback has to have a good understanding of all that. The more you do it, the more you get comfortable with it and the better you can execute it. I think that’s no different, really, than any other young quarterback or quarterback coming into a new system.”

In the Patriots’ 31-22 exhibition win over the Philadelphia Eagles on Friday night, Mallett completed 9 of 18 passes for 97 yards. Tebow went 4 for 12 for 55 yards with three sacks and ran four times for 31 yards.

Brady completed 7 of 8 passes for 65 yards and one touchdown and left after the first two series.