EAST RUTHERFORD, N.J. — New York Jets quarterback Geno Smith is day-to-day with an ankle injury that occurred Wednesday and resulted in him being scratched from the preseason opener Friday night against the Tampa Bay Buccaneers.

“It’s nothing to be alarmed about,” coach Aaron Glenn said after a 24-16 loss at MetLife Stadium.

Glenn wouldn’t characterize the injury, saying only that it’s “sore.” It happened in the second joint practice with the Bucs, though Smith didn’t miss any reps and talked enthusiastically afterward about making his debut.

Exercising caution, Glenn informed Smith on Thursday that he wouldn’t play.

The injury provided a glimpse of what life would be like without the veteran starter. The Jets don’t have a seasoned backup, so rookie Cade Klubnik got the nod against the Bucs. He joined J.J. Jones (1975) and Zach Wilson (2021) as the only rookie quarterbacks in franchise history to start the preseason opener, according to the Jets.

Klubnik, a fourth-round pick from Clemson, strengthened his case to be the No. 2 quarterback. In only two series, he led the starting offense to a field goal and a touchdown — a 31-yard run by Braelon Allen. Facing the Bucs’ backups, he completed 5 of 7 passes for 56 yards, including two third-down conversions.

“Although I’m a young player, I’m poised,” Klubnik said. “I don’t get flustered by big games and big moments.”

Klubnik had no practice reps with the starters, so his ability to adjust quickly was impressive. Two of his completions went to Garrett Wilson, who made a twisting, midair adjustment on a pass behind him — a 21-yard gain on third down.

Glenn lauded Klubnik for his poise.

“When we watched him in college, he was the same way. Nothing changes,” the coach said. “He has a moxie to him. He’s very confident. He’s confident in his arm. He’s confident in his ability to elude and make plays with his feet. And his ability to be able to come out and operate with the first group the way he did, to me I thought it was outstanding.”

Glenn said Klubnik did make some mistakes, adding, “Today was really a thumbs-up for him, but listen, guys, don’t get it twisted. We’re still in the competition. It’s just the first preseason game. And those guys can continue to fight it out.”

Klubnik is battling Bailey Zappe and Brady Cook for the No. 2 job. Zappe, who replaced Klubnik, hurt his chances with two interceptions, including a pick-six. Cook wasn’t impressive in mop-up duty and is running fourth on the depth chart.

Based on practice and the first game, Klubnik is the clear front-runner.

“I tell you what,” Glenn said, “I was happy and proud of the way he operated.”