Kellen Winslow Jr.’s use of a motorcycle might have violated standard NFL contract language that prohibits players from engaging in dangerous activities.

The Cleveland Browns could require Winslow, who lost millions last year because of a season-ending injury, to pay back a $4.4 million signing bonus he received in March.

The 21-year-old tight end sustained internal injuries and damage to his right shoulder and right knee in an accident that sent him flying over the handlebars of his newly purchased motorcycle.

Winslow was being treated at the Cleveland Clinic, where the team’s medical staff was waiting for swelling in the shoulder and knee to subside. The team declined Tuesday to provide an update on his condition.

NFL spokesman Greg Aiello said the standard NFL contract forbids players from taking part in any activity “which may involve a significant risk of personal injury.”

It’s another setback for the struggling Browns and Winslow, who entered the league last season with high expectations and a reputation as a hothead. His rookie season ended in disappointment when he broke his right leg during an onside kick in a Week 2 loss against Dallas.

With his leg still on the mend, Winslow purchased a sport bike last month and was learning how to ride it in a parking lot Sunday night when he hit a curb and was thrown off. He complained of chest pains after the crash.

Winslow held out last year before signing a $40 million contract _ a record for a tight end _ as the No. 6 pick in the draft.

Teams have the option of making contract language regarding dangerous activities more or less restrictive during negotiations with players, Aiello said. The Browns wouldn’t release information about Winslow’s contract.