The virus will keep the Jets quarterback sidelined during a brutal stretch in the team’s schedule.

FLORHAM PARK, N.J. — A new coach, a marquee running back and a second-year quarterback brought renewed hope to the Jets this season. But one week in, that optimism has quickly diminished.

Sam Darnold has mononucleosis and will not play on Monday night against the Cleveland Browns (0-1), Coach Adam Gase announced Thursday. There is no timetable for his return.

Darnold had received a diagnosis of strep throat Tuesday night, but further tests revealed he had mono. Trevor Siemian, who signed a one-year deal with the Jets in March, will start in his place Monday night.

“Anytime you lose your quarterback, it’s a big deal,” said Dowell Loggains, the team’s offensive coordinator and quarterbacks coach. “It is a next man up mentality. Trevor has been here the whole off-season. We’re excited about him.”

Siemian, 27, has not played in an N.F.L. regular season game since Dec. 14, 2017. He is 13-11 with 30 touchdowns and 24 interceptions in his 24 career starts, all with the Denver Broncos.

“I’ve been there and done it a little bit,” Siemian said. “It’s been a while. So, like I said, I’m chomping at the bit to compete and get out there with the guys. I feel comfortable.”

Monday’s matchup is the start of a tough stretch in the schedule for the Jets. After Cleveland, the Jets will travel to Foxborough, Mass., to face the Patriots in Week 3, followed by a bye week. They will then play at the Philadelphia Eagles before returning for home games against the Dallas Cowboys and Patriots.

An early rash of injuries has only made things more difficult. Le’Veon Bell, the star running back signed by the Jets this off-season, did not practice Thursday because of a sore shoulder, but he later announced by Twitter that a magnetic resonance imaging exam had shown that the shoulder was fine. Demaryius Thomas, acquired on Monday in a trade with the Patriots, is dealing with a hamstring injury and will not practice until Saturday. Linebacker C.J. Mosley, the leader of the defense, left Sunday’s game with a groin injury, and the rookie defensive end Quinnen Williams, who was drafted third over all, left with an ankle injury. Neither player has practiced this week.“It’s a lot of adversity early on, but it’s going to make us closer as a group, as a staff, as an organization,” safety Jamal Adams said. “There’s no better way to do it than on Monday night.”

Gase and the Jets had hoped to rebound from a week 1 loss to the Bills in which they gave up a 16-0 second-half lead at home. After the game, Gase was straightforward about his disappointment with the way his wide receivers played.

“We were frustrated,” Gase said during a conference call Monday. “We lost a game. We watched the film, mistakes were made, there were some things that I wish I would have done different; I voiced that part to those guys.”

The receiving corps has also been hit by injuries. Quincy Enunwa is out for the season with a neck injury. They are also without tight end Chris Herndon, who has been suspended for four games for violating the league’s substance abuse policy. The Jets hope that Thomas, a five-time Pro Bowler, can provide a lift when he is healthy enough to play.

Darnold, Gase said, was not sick during Sunday’s game. The coach sent Darnold home before Wednesday’s practice started because, Gase said, Darnold did not look good. Gase and members of the Jets’ training staff went to Darnold’s house Wednesday night to tell him the news.

“He moved past the bad news and went into ‘All right, how am I going to get myself ready?’” Gase said.

Siemian played with the first-team offense at Thursday’s practice. He has a calmness about him, Gase said, and he does not panic in a tight situation. Gase said Siemian was the “perfect guy” to back up Darnold.

“We signed Trevor specifically because of his 24 starts over his career and a winning record,” Gase said. “He has a lot of experience, he’s been in a lot of ball clubs. He has the respect of the locker room.”

Gase said he told the rest of the team about Darnold at a meeting Thursday morning. Darnold’s locker remained untouched after practice, his jersey hung neatly next to the rest of his belongings, the way it will remain for the forseeable future.