Jets coach Adam Gase announced Tuesday that backup quarterback Trevor Siemian will require season-ending surgery on his ankle, and third-stringer Luke Falk will be the team’s starter until Sam Darnold returns from his bout with mononucleosis.

Siemian, who signed a one-year deal with the Jets in the offseason after making 25 career starts with the Broncos, had a disastrous debut in Monday night’s 23-3 loss to the Browns, in which he completed just 3-of-6 passes for six yards, and exited the game midway through the second quarter, following a vicious hit by Myles Garrett. When Siemian fell to the ground, his left leg bent backwards, resulting in ligament damage.

“It’s an ugly-looking play,” Gase said. “I think there’s a good amount of damage in there to that surgery will be needed and it’s gonna be time for him to recover.”

Falk, who received his first-ever regular season in action in relief of Siemian, completed 20-of-25 passes for 198 yards and will make his first career start Sunday in New England.

Gase said the team hadn’t yet decided who will be brought in to back up Falk, a 2018 sixth-round pick, who’d previously been waived by the Titans and Jaguars. The coach had no update on a timeline for Darnold’s return.

“We’ll really have a better idea when doctors give us the green light he can progress,” Gase said of Darnold. “He was in the building today. He’s feeling better. The doctors are letting us know when to proceed with him.”

Gase also said that linebacker Jordan Jenkins will be week-to-week (calf), along with wide receiver Demaryius Thomas (hamstring/knee).