Quarterback Sam Darnold is out indefinitely with mononucleosis and will not play Monday night when the New York Jets host the Cleveland Browns.

Darnold could wind up missing more than just one game, coach Adam Gase said, and has already lost weight. The Jets play the New England Patriots in Week 3 before a bye the following week. “(No.) 14 has mono and will be out for this game,” Gase said Thursday. “So Trevor (Siemian) will be starting. Luke Falk will be elevated (from the practice squad) at some point and be the backup.”

Le’Veon Bell played his first NFL game in 20 months last week and the running back appeared to hold up fine. Gase said Bell didn’t practice Thursday because of a sore shoulder, and ESPN’s Adam Schefter reported Bell was scheduled for an MRI exam. ESPN’s Jeremy Fowler reported Thursday afternoon the exam was “mostly precautionary” and didn’t show any major damage or tears.

Also not practicing are inside linebacker C.J. Mosley, who injured his groin in Sunday’s loss to the Buffalo Bills, and safety Jamal Adams (hip). Defensive lineman Quinnen Williams (ankle) was also listed as not practicing for the Jets, although his injury is not considered serious.