BEREA, Ohio -- The Cleveland Browns do not believe quarterback Baker Mayfield's left shoulder injury will end his season or require immediate surgery at this point, a league source told ESPN.

Mayfield, however, is still waiting on a second opinion from his own doctor. That doctor is expected to review his MRI scans later Tuesday.

Mayfield initially suffered the shoulder injury -- a partially torn labrum -- on Sept 19 while attempting to make a tackle after an interception. He continued to play through it, wearing a harness on the shoulder for support. But Mayfield aggravated the shoulder injury Sunday against the Arizona Cardinals.

During the third quarter, Cardinals pass-rusher J.J. Watt drove Mayfield's left shoulder into the ground, resulting in a fumble. Mayfield stayed down for several minutes and had to be helped off the field, though he didn't miss a snap.

After the game, Mayfield, who was wearing a sling, said his shoulder had popped back out earlier on the drive during a noncontact play.

Still, Mayfield vowed to play in Thursday night's game against the Denver Broncos. The Browns are scheduled to practice Tuesday. If Mayfield can't go, backup Case Keenum will get the start in his place.