Mitch Trubisky was under center when Packers linebacker Clay Matthews realized he needed to get off the field Sunday night.

Fearing a flag for 12 men on the field — had the Bears snapped the ball, they would’ve converted their third-and-one on the first drive of the fourth quarter — Matthews sprinted for the sideline.

He made it in time.

Trubisky didn’t see him. He sent tight end Trey Burton in motion. The Bears ran a misguided pass play that gained exactly zero yards, were forced to punt and watched Packers quarterback Aaron Rodgers throw the second of three fourth-quarter touchdown passes in a crushing 24-23 loss.

Coach Matt Nagy spent all preseason preaching patience. The third-and-one failure encapsulated the Bears’ growing pains in one tidy play.

A more veteran quarterback could’ve called for the snap and drawn the penalty. A more veteran coach than Nagy, who was making his debut, might’ve called a different play. Running back Jordan Howard, who averaged 5.5 yards per carry, surely could’ve plunged for a yard.

Instead, Trubisky — who didn’t have the authority to change that particular play — ran play action right, planted his feet and threw back across the field to Dion Sims. The only pass-catching option on the play, Sims ran his crossing route too shallow for a first down. It didn’t matter. Packers safety Ha Ha Clinton-Dix sniffed it out.

Nagy and Trubisky expected the Packers to play man defense. Had they, Trubisky surmised after the game, “We look like geniuses.”

Instead, they looked like the opposite.

“There’s gonna be times, and you go back and forth, always looking at yourself — What could you have done better? What could you have called better?” Nagy said Monday. “There’s definitely some in there that you look at yourself, I look at myself, and say, ‘Hey, should I have done this or should I have done that?’ . . .

“Hindsight’s 20/20. You look back and you say, ‘Oh, I wish we would have run the ball there.’ But we didn’t.”

Matthews sprinting for the sideline was obvious on coaching film, Nagy said. In the moment, though, Trubisky was focused on the cadence and the play clock.

“It’s not as easy when you’re down there on the field,” he said. ‘‘But if he knew that, we’d snap the ball and get the free first down.”

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Nagy’s decision to sit the Bears’ starters in their fourth preseason game, while prudent from an injury-prevention standpoint, ran counter to his notion that every snap mattered.

He said two weeks ago that 25 Trubisky snaps wouldn’t be the difference between a win or a loss in Green Bay. He reaffirmed that Monday — despite the score — by pointing to success in the Bears’ first two drives, which resulted in 10 points, and sustained series of 12 and 14 plays in the second half that yielded field goals.

“Yeah, I stand by that,” he said. “That’s, again, with what we did [Sunday]. It would have been different if we would have just come out and not had any of those extended drives.”

Nagy said he was pleased with how Trubisky, who went 23-for-35 for 171 yards, handled the team in the huddle and with his footwork in the pocket.

“He played the way I knew he could play,” Nagy said. “He made some plays. Now we left some out there, too, but that’s gonna happen. Shoot, there’s some plays that [Rodgers] left out there, too — other than the second half.”

He smirked, painfully.

“For the most part, Mitch is going to learn,’’ Nagy said. ‘‘He’s going to keep growing. He understands that. First thing he said to me on the bus was, ‘How can I get better?’ That’s the best part about him and where we’re at right now.”

And, after Sunday, the best hope the Bears have.