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Panthers quarterback Cam Newton has a hairline fracture in one of his ribs that has left his availability for the start of the regular season in question.

Panthers coach Ron Rivera said an MRI exam on Sunday morning revealed the fracture in Newton’s lower back.

Rivera said Newton was out for Thursday’s exhibition finale at Pittsburgh, and Rivera expects him to play in the season opener at Tampa Bay on Sept. 7. But Rivera said the team won’t be sure of Newton’s status for Week 1 until next week.

Rivera said Newton was “very, very sore” when he arrived at Bank of America Stadium on Sunday morning, so the medical staff sent him for tests.

“He won’t play this week. ... We’ll treat him day by day and evaluate him at the end of the week and see how he is,” Rivera said. “We expect him to get past the soreness in the next few days and then be able to do a lot more as the days go. And we’ll see how it all unfolds.”

Newton was injured during the second quarter of Friday’s night loss at New England when Patriots linebacker Jamie Collins kneed Newton in the back while making a tackle at the end of Newton’s 7-yard scramble.

Newton was wearing rib protectors, but Rivera said Newton was struck in the crease of the protectors.

“Unfortunately what happened was, the way the (rib protector) sits, that was the only crease in the protection. And wouldn’t you know it, that’s the one spot he got it on,” Rivera said. “I just happened to be in the locker room when they brought him in at halftime and watched it and said, ‘Where’d you get hit?,’ and he pointed right between the two shells. It’s crazy how that happens but that’s what happened.”

Newton has not missed a regular-season start in his first three NFL seasons and has been sidelined for only a handful of snaps due to injury.

But Newton was in obvious pain Sunday when he walked on to the Panthers’ practice field with head athletic trainer Ryan Vermillion.

Newton has not been available to reporters since getting hurt Friday night. Through a team spokesman, he deferred questions to Rivera following Sunday’s practice.

A sports neurologist and pain management specialist said the biggest obstacle for Newton will be dealing with the pain.

Vernon Williams, a sports neurologist at Kerlan-Jobe Orthopaedic Clinic in Los Angeles, said it sounds as though Newton sustain an “uncomplicated fracture” with no displacement of the bone.

Williams said the recovery timeline described by Rivera was accurate.

“Normally you can get the pain controlled, and I wouldn’t expect him to be out for an extended period of time,” Williams said. “Now the rib might still be healing, but I would expect within a week or so he’d be ready to return with proper management.”

In addition to rest, the Panthers can treat Newton’s pain with medication, ice, analgesic creams or by injecting him with a numbing agent, Williams said.

Newton’s injury comes at a bad time for the Panthers. Rivera has been concerned with the timing between Newton and the team’s top receivers, all of whom are in their first season with the Panthers.

Backup quarterback Derek Anderson’s wife is expecting their first child any day, and fourth-team quarterback Matt Blanchard is sidelined after sustaining a concussion against the Patriots.

Third-team quarterback Joe Webb was signed as an emergency option following Newton’s offseason ankle surgery. But Rivera said Webb’s chances of making the team now have greatly increased.

Newton, 6-foot-5 and 245 pounds, has been hit more than any quarterback over the past three seasons and underwent surgery in March to have the ligaments tightened in his left ankle.

The Panthers have been cautious with Newton since the surgery. He missed all but the final practice during organized team activities. During training camp at Wofford, Newton was restricted from running any read option plays until the final week.

But Newton was sacked three times against the Patriots and scrambled three other times. On the play in which Newton was hurt, running back DeAngelo Williams was running open down the sideline, but Newton didn’t see him.

Newton ran out of the pocket, went down awkwardly and rolled into Collins’ knee.

“The unfortunate part is he had a chance to make a play on it. He chose to tuck it and run. And one of the things he’s going to have to learn and understand is either dump it or learn how to slide,” Rivera said. “... That’s him. That’s just who he is. He tries to get everything out of it, and it’s just unfortunate. One of those fluke things that happened.”