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Carolina quarterback Cam Newton left the Panthers’ preseason game Thursday at New England, and tests Friday revealed a mid-foot sprain that casts doubt on his availability for Week 1 of the regular season.

GM Marty Hurney said Friday that the team was “cautiously optimistic” Newton would be ready for the Sept. 8 opener, per NFL Network.

Newton was hurt when he was sacked after scrambling in the pocket, during his third series of the game. He limped off the field, and trainers looked at his foot on the sideline before taking him back to the locker room.

The team announced shortly afterward he would not return, and he left the stadium after the game in a walking boot. Head coach Ron Rivera told reporters he did not know the severity of the injury.

Hurney told the team’s radio network after the game, “We’re hoping it’s not too bad. … We’ll find out more tomorrow. But I hope it’s not too bad.”

Newton was making his preseason debut, his game first action since having right (throwing) shoulder surgery in the offseason. He went 4 of 6 for 30 yards, taking two sacks, before departing.

Backup Kyle Allen replaced Newton.

Newton, 30, is entering his ninth NFL season. The NFL MVP in 2015, last season he completed 67.9 percent of his passes for 3,395 yards, 24 touchdowns and 13 interceptions in 14 games, missing the final two due to the shoulder injury.

The Panthers open the regular season Sept. 8 against the visiting Los Angeles Rams.

–Field Level Media (@FieldLevelMedia)