Adam Gase accepted a job with the belief the same quarterback could start every game for the next decade or more. Instead, the Jets’ first-year head coach will turn to his third different starting quarterback in the first three weeks of the season.

Sam Darnold is sidelined with mononucleosis, and Gase announced Tuesday backup quarterback Trevor Siemian will require season-ending surgery on his left ankle, before the coach anointed third-stringer Luke Falk as the team’s starter for Sunday’s game at New England. Gase said the team was in the process of determining who would be brought in to back up Falk — who had never thrown a regular-season pass before Monday — with free agents and former Gase quarterbacks Brock Osweiler and David Fales both possibilities.

Gase said Falk will remain the starter until Darnold comes back. The franchise quarterback returned to the team facility Tuesday and believes he should be able to play for the Week 5 matchup against the Eagles, meaning he might miss just one more game (the Jets have a bye in Week 4).

“That’s what the doctors are estimating,” Darnold said on ESPN’s “The Michael Kay Show.” “I’m feeling a lot better… The worst thing about this sickness, now that I’m feeling better, I still can’t do anything. That’s the worst part about this. And the timing couldn’t be worse either.”

Darnold watched Monday night’s 23-3 loss to the Browns at home with his parents and erupted when he saw Siemian — who had signed a one-year deal with the Jets and completed 3-of-6 passes for 6 yards in his lone start — taken down by Myles Garrett, who snapped the quarterback’s left leg backward, causing ligament damage.

“When it initially happened, I actually had a little episode. I went to the room and started screaming into a pillow and punching my pillow as hard as I could. It was tough,” said Darnold, who spoke with Siemian on Tuesday. “He said it was one of the most painful things that he’s ever been through.”

Darnold, who revealed he was also suffering from strep throat last week, said he’s lost only 2 pounds since contracting mono, and still isn’t sure how he got sick.

“I know there was a sign also at the game that some girl claimed she gave it to me, but I’ve never seen that girl in my life. I’d like to set the record straight on that,” Darnold said jokingly. “It’s one of those diseases that’s really tricky.

“I feel great. I feel amazing. My energy’s back up. … I feel fine. I gotta wait this thing out. I gotta wait for my spleen to get back to normal.”

Until then, the Jets will rely on Falk, a 2018 sixth-round pick who previously had been cut by the Titans and Dolphins, where he played under Gase. After spending the first week of the season as part of the practice squad, Falk was promoted to the backup spot and came in Monday for Siemian, completing 20-of-25 passes for 198 yards in his first NFL game.

“We have all the confidence in Luke,” Darnold said. “He played pretty well when he came in. He was throwing pretty good balls, hitting guys in stride. He knows the offense really well.”

Despite the looming matchup with the defending Super Bowl champions, Falk felt he’d be even better with a full week of practice.

“I haven’t been getting any reps in the sense of physical practice, you’re just trying to get the most you can mentally,” Falk said following Monday’s loss. “I think definitely having a week of preparation and a week of practice, I’ll be ready to roll.”

Darnold expects to take back the reins soon afterward.

“I’m confident in our team to be able to rally,” Darnold said. “If I get back for Philly and we go on a little run, obviously we got our work cut out for us … but anything’s possible.”