Adam Gase walked into Thursday’s press conference and warned reporters of bad news.

“Is the building burning down?” he was asked.

“Close,” the first-year Jets coach responded.

There was no fire to be found other than the embers of this once promising season.

Sam Darnold won’t be on the field Monday night against the Browns, and he likely won’t be there in Week 3 against the Patriots, either. The Jets’ franchise quarterback has been diagnosed with mononucleosis and will be replaced by backup Trevor Siemian. Luke Falk will be elevated to be Siemian’s backup.

“It looks like [it will be more than one week],” Gase said. “Good thing we got an early bye week. … I know he’s out this week, and past that I’ll have more information as we go.”

When pressed about a timeline, Gase said: “I don’t want to put a prediction on anything. I’m just glad we got the bye week when we do.”

The Post spoke to two doctors who provided insight into what Darnold is dealing with. Dr. Charles Popkin, an assistant professor at Columbia University Medical Center and orthopedic surgeon familiar with the illness, said it usually takes someone two to three months to fully heal from mono, though Darnold may be able to return before then.

Dr. David Chao, the former Chargers team physician, believes Darnold’s timeline will begin at four to six weeks. Mono typically causes the spleen and/or liver to enlarge, and a person can’t have physical contact because of the risk of rupturing the organs, according to the doctors. Darnold will feel tired and lose weight, symptoms that could last multiple weeks. He also can’t be around the team because he could infect others.

The Jets initially thought Darnold was just battling strep throat. But when antibiotics weren’t working as expected, he underwent a series of tests that revealed the illness. Darnold has already lost some weight — Gase estimated five pounds — and will be resting and undergoing further tests.

When Gase, accompanied by offensive coordinator Dowell Loggains and trainer Dave Zuffelato, broke the news to Darnold in person late Wednesday night, he didn’t take it well, the coach said. But disappointment quickly turned to what he can do to get better and not lose too much weight.

“I wanted to make sure it was delivered right and I figured I could be the bearer of bad news on this one,” Gase said. “He went right into, ‘[What are] my solutions?’ He moved past the bad news and went into, ‘All right, how am I going to get myself ready for when I’m ready to return, so I’m not in a bad place?’ ”

Gase tried to put a positive spin on the news, waxing poetic about Siemian and his calm demeanor. The 27-year-old signal-caller has started 24 games in his career, all with the Broncos in 2016 and 2017. He spent last year with the Vikings backing up Kirk Cousins. Gase broke the news to the team Thursday before practice.

“We talked about how our next-man philosophy does not waver. It’s about everybody doing their job. That’s why Trevor’s here,” Gase said. “We signed Trevor specifically because he has 24 starts for his career with a winning record. He has a lot of experience. He has been on winning ballclubs. He has the respect of the locker room.

“The reaction I got, and the way that guys took it, was let’s go. There’s confidence there. I think we’re going to look back at this and it’s going to be a good thing for us. It’s going to make that locker room even tighter.”

The players offered a similar sentiment. The season isn’t over, despite the loss of Darnold and the unforgiving upcoming schedule. They’re just facing adversity early on.

“It’s going to show us what we’re made of,” safety Jamal Adams said. “I actually like it. There’s no better way to [bounce back] than on Monday night.”

— Additional reporting by Brian Costello