EUGENE, Ore. — One moment, UCLA backup quarterback Ty Dieffenbach is watching from the sideline, not having thrown a pass all season.

The next, he catches his name being called to replace injured starter Nico Iamaleava in the Bruins’ biggest game in years.

“When I heard that,” Dieffenbach said, “I was just tunnel-visioned and ready to go execute.”

This wasn’t just any opportunity. Dieffenbach entered a nationally televised showdown against Oregon in one of the most intimidating environments in college football.

The onetime third-stringer didn’t feel any nerves as he stepped onto the field before a sellout crowd at Autzen Stadium.

“I’ve been waiting for this moment,” he said. “I’m ready.”

Was he ever.

In a gutsy performance for the ages, Dieffenbach helped the No. 21 Bruins hold off No. 13 Oregon, 17-15, on Saturday afternoon.

Playing the entire second half, the transfer from Cal Poly threw the fourth-quarter touchdown pass that provided the decisive score and effectively managed the offense to help his unbeaten team notch a momentous triumph.

Dieffenbach leapfrogged fellow backup Madden Iamaleava to earn the opportunity, coach Bob Chesney explaining afterward that Dieffenbach had put together an excellent week of practice and was a better option to run the ball in a game where the Bruins needed his legs.

His performance provided another example of Chesney’s mantra that players are either exposed or revealed when someone goes down and a teammate must step up.

“For Ty to step up in that moment when Nico couldn’t go,” Chesney said, “he put himself in a spot where he was revealed for knowing the run game, knowing what to get us into, the touchdown pass.”

That revelatory moment came early in the fourth quarter with the Bruins (5-0 overall, 3-0 Big Ten) holding a 10-6 lead and needing another score to create some breathing room.

On third-and-3 at Oregon’s 9-yard line, Dieffenbach rolled out and fired a pass to running back Wayne Knight, who was isolated on a slower defender. Knight caught the ball and ran into the end zone for what proved to be the winning points.

Dieffenbach’s numbers were hardly flashy. He completed three of six passes for 43 yards and a touchdown. He ran for six yards on one play but was also brought down behind the line of scrimmage a few times.

But his assured presence in the huddle and his mastery of the biggest moments rubbed off on teammates who needed a leader after Nico Iamaleava went down late in the second quarter with what was described as a leg injury.

“His ability to come in a game like this and an environment like this and just play as poised as he did and keep the team moving and the way he did,” said Knight, who finished with 103 yards rushing in addition to his touchdown catch, “it just shows a lot about his character the kind of guy he is.”

Immediately showing his fearlessness, Dieffenbach nearly threw for a touchdown on his first pass late in the second quarter. It was on the mark, but wide receiver Brian Rowe Jr. was unable to come up with the ball in the corner of the end zone.

Iamaleava returned for one more play, throwing a pass that was intercepted in the end zone, before sitting out the rest of the game.

“He wasn’t close to being able to come back today,” Chesney said. “I thought he got hit in the thigh, there might be a little bit more to it, which we’ve got to figure out as we go through the process and evaluate him.”

Dieffenbach’s college resume before Saturday essentially consisted of one so-so season at Cal Poly, where he threw for 1,305 yards and nine touchdowns with nine interceptions.

He was recruited by UCLA offensive coordinator Dean Kennedy out of Agoura High when Kennedy was at Holy Cross. Dieffenbach spent his first two college seasons at Pittsburgh, never appearing in a game, before transferring to Cal Poly.

When Dieffenbach entered the transfer portal after last season, Kennedy invited him to become a Bruin.

“I was like, ‘Oh, let’s go, let’s go do this thing,’” Dieffenbach said. “It was a no-brainer.”

It paid off on a Saturday unlike any other.

Dieffenbach said he had envisioned this kind of moment in conversations with his father, Daniel, who was in the back of the interview room as he spoke with reporters.

“It was just great, man,” Dieffenbach said, “and it was a big one for my family, for my hometown, and for everybody.”

Especially for the team that remained unbeaten, with unlimited possibilities beckoning.