Panthers coach Ron Rivera is hesitant to play Newton due to the rib injury he suffered in a preseason game last month against the New England Patriots, sources told Mortensen.
If Newton doesn't play, Derek Anderson would be Carolina's likely starter against Tampa Bay.
Rivera previously had said that Newton, who was a full participant in Friday's practice, would be a game-time decision for Sunday's contest, which starts at 4:25 p.m. ET.
"There's some things you can't measure, and that's how he'll react when he gets hit," Rivera said Friday. "So when we make our decision Sunday morning, it's going to be based on how we feel he's done the last couple of days, how he feels in terms of handling what we want to do with the game plan and whether or not we believe he'll be able to protect himself if he takes a shot."
Newton fractured his ribs during an Aug. 22 exhibition game at New England. He did not practice Wednesday and was limited Thursday but did take all the repetitions during installation of the game plan.
ESPN.com Panthers reporter David Newton contributed to this report.