Rob Ninkovich is surprised by Sam Darnold's mononucleosis diagnosis but expects the Jets QB will recover quickly. (1:22)

FLORHAM PARK, N.J. -- 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.

The Jets, who lost starting wide receiver Quincy Enunwa to a season-ending neck injury this week, could be without Darnold for multiple weeks.

"Looks like it," Gase said. "Good thing we've got an early bye week."

The bye week falls in Week 4, following an away game against the New England Patriots. Darnold's earliest likely return is Week 5 at the Philadelphia Eagles.

Darnold isn't the only concern on offense.

Running back Le'Veon Bell underwent an MRI on Thursday to evaluate shoulder soreness, though a source told ESPN's Jeremy Fowler that no major damage or tear was found.

Gase didn't sound alarmed about Bell, saying, "We don't think it's anything worse than [soreness]. We're just trying to be a little extra cautious."

Darnold received the diagnosis late Wednesday night, when Gase, offensive coordinator Dowell Loggains and a trainer showed up at his apartment near the Jets' facility.

"He knew something was up when Dowell and myself showed up," Gase said of his second-year quarterback, who didn't practice Wednesday because of what the team called strep throat. "I wanted to make sure [the news] was delivered myself. I figured I could be the bearer of bad news on this one."

Weight loss and extreme fatigue are the most common symptoms with mononucleosis, which has been known to last multiple months in some cases.

Darnold expressed concern about recent weight loss. He dropped 5 pounds, according to Gase.

"That was something that was alarming to him, that he lost some weight already in the last couple of days," Gase said.

The biggest concern with mononucleosis, medical experts say, is an enlarged spleen. A hit to the midsection over the next few weeks could rupture the organ.

Gase didn't provide any specifics with regard to Darnold's condition, saying he's still undergoing tests.

The Jets sent meals to Darnold's apartment because the team wants him to stay away from the facility due to the possibility of spreading the disease.

The team already has reached a pressure point in the season, coming off a crushing Week 1 loss to the Buffalo Bills in which the Jets blew a 16-point lead at home.

Darnold was lackluster in the opener, passing for only 175 yards, but he wasn't ill on Sunday, according to Gase.

Darnold's diagnosis, coupled with Enunwa's injury, will be costly for an offense that underachieved in the opener, managing only eight points.

Siemian, who signed as a free agent in the offseason, is 13-11 as a starter, but he hasn't played in a game since 2017 with the Denver Broncos. He spent last season with the Minnesota Vikings but didn't see any game action behind Kirk Cousins.

"I've played [in the league], so I guess there's a sense that I've been there and I've done it a little bit," Siemian said. "It's been a while. I'm champing at the bit."

Siemian, who worked exclusively with the backups in training camp and the preseason, said his mindset Monday night will be to "do my job, run a clean operation and get the ball in the end zone. It's pretty simple. ... I feel pretty comfortable with what we're doing. I'm excited to compete, for sure."

Luke Falk will be elevated from the practice squad to be the No. 2 quarterback.

Gase broke the news about Darnold to the players in Thursday morning's team meeting.

"We talked about our next-man philosophy does not waver," the coach said. "It's about everybody doing their job. That's why Trevor is here. We signed Trevor specifically because of his 24 starts with a winning record. He has a lot of experience and has been on winning ballclubs. He has the respect of the locker room.

"The reaction I got, and the ways guys looked, was, 'Let's go.' There's confidence there."

Caesars Sportsbook reopened with the Browns minus-4 and a total of 44.5. Before the Darnold news, the line had been Browns minus-2.5 with a total of 46. The line is as high as Browns minus-5.5 at other Vegas sportsbooks.