The prominent Manhattan surgeon found dead in his Upper East Side apartment Sunday with a knife sticking out of his chest was sued last year by a former Giants running back who claimed the doctor ended his football career.

Dr. Dean Lorich, who gained fame for treating U2 frontman Bono after his 2014 bike accident, performed surgery on Michael Cox, now 29, after he suffered a broken leg during a Nov. 9, 2014, game in Seattle, according to the ex-player’s lawyer, Steven North.

The doctor, who was associate director of the Orthopedic Trauma Service at the Hospital for Special Surgery, mended Cox’s fibula — but didn’t fix one of his damaged ankle bones, North alleged.

“We maintain that it should have been repaired before it got worse, and it wasn’t,” the lawyer said.

“Michael went on to see Lorich for a few post-operative visits, and the condition continued to deteriorate, apparently it wasn’t recognized and nothing was done about it.”

Cox, who had a four-year, $2.3 million contract with the Giants, will likely be unable to play football again, according to North.

“He went to [other] doctors, they noted a major difference in the deterioration of that bone, tried to salvage it [and] could not,” the lawyer said. “And he’s precluded in all likelihood from playing football again.”

Cox filed suit against Lorich, the Hospital for Special Surgery and New York-Presbyterian Hospital in May 2016, seeking unspecified damages.

“Lorich and his agents, servants and employees were negligent and careless by failing to provide proper medical and surgical, care, services, diagnosis, evaluation, assistance, and treatment to Michael Cox,” the lawsuit says.

On Sunday afternoon, Lorich, 54, was found by his 11-year-old daughter on the bathroom floor of the family’s Park Avenue home with a knife stuck in his chest, police said.

There were no signs of forced entry and police sources said it appeared to be a suicide.

North said Lorich was “in a very good reasonable mood” when he deposed him.

“I believe the hospital’s insurance covers [Lorich],” North said.

“I don’t see a connection between our lawsuit, which has been going on for quite some time, and his unfortunate demise. But you don’t know what goes through people’s mind.”

Lorich performed surgery on Bono after the singer lost control of his bike in Central Park while swerving to avoid another cyclist. He landed on his face and suffered multiple injuries, including a shoulder fracture.