Denver Broncos quarterback Brandon Allen warms up before the start of an NFL football game between the Denver Broncos and Green Bay Packers Sunday, Sept. 22, 2019, in Green Bay, Wis. (AP Photo/Matt Ludtke)

ENGLEWOOD | It took Brandon Allen four seasons and three different teams to get his shot at starting in the NFL.

The gig may only be temporary, even if Joe Flacco is out for a while with a herniated disc in his neck.

Rookie quarterback Drew Lock could return to the active roster any time now that he’s missed half the season after going on injured reserve with a right thumb injury suffered in the preseason.

Allen’s not looking over his shoulder but straight ahead to Sunday and his debut when the Denver Broncos (2-6) host Baker Mayfield and the Cleveland Browns (2-5).

“This is the opportunity that everybody wants, everybody strives for,” Allen said Tuesday. “I’m going out to do the best I can do.”

Flacco, who was sacked 11 times in his last two games, had a stiff neck after Denver’s 15-13 loss at Indianapolis last weekend and coach Vic Fangio said Monday that Flacco was diagnosed with a herniated disc in his neck. It’s an injury that could sideline him for a significant stretch and land him on injured reserve.

“We’re feeling for Joe right now, but this is the opportunity you work for,” said Allen, who was claimed off waivers from the Rams on Sept. 1. “I’ve learned a lot from him in the few short weeks we’ve been together.”

Broncos fans are brushing up on Allen, a sixth-round pick of Jacksonville in 2016. He spent ’17 and ’18 with the Rams.

This is what the University of Arkansas product wants everyone to know about him: “They’re going to see me play hard. They’re going to see I want to win and that’s kind of the biggest thing I want them to take away.”

Allen will be the Broncos’ sixth starting quarterback since Peyton Manning retired. It will be Allen’s first start since guiding Arkansas past Kansas State 45-23 in the Liberty Bowl on Jan. 2, 2016.

His backup? TBD.

It could be Lock, the second-round pick out of Missouri who sprained the thumb on his throwing hand in a preseason game against San Francisco on Aug. 19 and hasn’t practiced since. Or, more likely, practice squader and undrafted rookie Brett Rypien of Boise State, the nephew of longtime NFL quarterback Mark Rypien.

“I’m just trying to come in and do my best every day,” Brett Rypien said.

Lock said his thumb was cleared about two weeks ago. He’s been traveling with the team, taking in meetings and going through virtual-reality simulations.

“It’s kind of fun to be able to go about things, kind of my own way a little bit,” Lock said. “Whenever they’re ready, I’ll be ready. They’ll get a confident guy whenever they call the number out.”

Allen feels prepared even if he’s only taken a handful of snaps with the first-team offense. The Broncos’ structure is similar to the one he learned while with the Rams. He said he can handle the full playbook, too.

The 6-foot-2 Allen doesn’t have the height of Flacco (6-6) but makes up for it in mobility.

“I can escape the pocket and make plays outside the pocket,” the 27-year-old said.

Dan Enos was the Razorbacks’ offensive coordinator and quarterbacks coach in 2015 when Allen threw for 3,440 yards and 30 touchdowns his senior year.

Enos knows exactly what the Broncos are receiving.

“Brandon is a competitor. He’s very smart,” said Enos, now the offensive coordinator at the University of Miami. “He was a guy who, when I coached him, if he made a mistake once, he didn’t make the same mistake again. As a coach, that’s one of the things you look for in a guy — someone who doesn’t repeat mistakes. Brandon didn’t do that. He learned from mistakes.”

Enos recalled a time the Razorbacks lost to Alabama after leading at halftime. They were 2-4 going into their bye week. The two had a conversation about what Allen wanted his legacy to be.

Allen’s response stuck with Enos.

“He said, ‘Coach, I just want to help the team win. … I don’t care about how many yards I throw for or any of that,'” Enos said. “I saw a guy that, after that, he really put the team on his back. We went and beat Auburn. We beat LSU at LSU, we beat Ole Miss at Ole Miss. … Brandon played out of his mind the rest of the season.

“The whole team really fed off of his play.”

NOTES: Broncos LB Corey Nelson is expected to be placed on injured reserve with a bicep injury.