Cam Newton's surgically repaired shoulder has been the focus for much of the Carolina Panthers' preseason, but the focus turned to the franchise quarterback's left foot and ankle after a first-quarter sack in Thursday night's 10-3 preseason loss at New England.

Team medical staff examined Newton's left foot on the sideline after the sack, his second in three series. The 2015 NFL MVP then walked to the locker room, and the team later announced he would not return.

A league source told ESPN that an initial diagnosis on Newton, who was in a walking boot after the game, was a sprained ankle, and that more will be learned Friday with further tests.

"We're hoping it's not too bad," Panthers general manager Marty Hurney told the team radio network regarding Newton. "We'll find out more tomorrow. But I hope it's not too bad."

Newton, 30, underwent surgery on his left ankle in March 2014.

Newton sat out the first two preseason games as the staff used extreme caution in his recovery from January arthroscopic surgery on his throwing shoulder, the second time the right shoulder had been operated on in three offseasons.

He completed his first four passes Thursday for 22 yards before his first incompletion, a throwaway to avoid a sack.

Newton didn't get rid of the ball on the play he was injured. He scrambled around the pocket before going down with Patriots defensive lineman Adam Butler holding on to his left foot.

Replays didn't show anything out of the ordinary, but Newton was favoring the foot when he left the field.

This wasn't the first time Newton incurred a preseason injury against the Patriots on a scramble. He suffered a fractured rib in 2014 and ultimately missed the opener at Tampa Bay.

Newton finished Thursday's game 4-for-6 for 30 yards.