New York Jets quarterback Sam Darnold will be sidelined indefinitely after being diagnosed with mononucleosis, coach Adam Gase announced Thursday.

Trevor Siemian will start Monday night against the Cleveland Browns.

Gase hinted that Darnold could miss several weeks. He suggested the earliest likely return on Week 5 at the Philadelphia Eagles, following the Jets’ Week 4 bye. They play at the New England Patriots in Week 3.

“I don’t want to put predictions on anything,” Gase said, when asked about a specific timeline for Darnold’s return. “I’m just glad we’ve got the bye week when we do.”

Gase and a team trainer and quarterbacks coach Dowell Loggains broke the news to Darnold late Wednesday night.

Darnold didn’t practice Wednesday because of what the team called strep throat.

Gase said Darnold already has lost weight because of the mono. Luke Falk will serve as the Jets’ backup quarterback while Siemian is the starter.

Gase also announced that star running back Le’Veon Bell will undergo an MRI on his shoulder Thursday and will miss practice.

The Browns were favored by 2.5 points against the Jets at Caesars Sportsbook before the news broke on Darnold. The game is now off the board.