Only once in Mike Zimmer's four seasons as Vikings coach — and only twice in this decade — has the team had a quarterback start all 16 regular-season games. And when the Vikings opted to let all three of their veteran quarterbacks hit free agency this spring, they knew two things:

They needed to sign Kirk Cousins. And they needed to have a reliable backup.

Enter Trevor Siemian, whose trade from Denver to Minnesota technically came together a day before the Vikings signed Cousins to a three-year, $84 million deal. Siemian became the first Broncos quarterback to start a game after Peyton Manning, leading the Broncos to a 21-20 victory over the Panthers in a rematch of Super Bowl 50 to open the 2016 season.

He threw for 3,401 yards and 18 touchdowns that season, going 8-6 in his 14 starts before losing his job in 2017. He became expendable in Denver in March when the Broncos signed Case Keenum — the former backup who led the Vikings to the NFC Championship Game, made himself rich in the process and delivered another reminder of how important a commodity a backup quarterback can be.

So while the Vikings have pinned their Super Bowl hopes to Cousins in many ways, they're also grateful for an understudy who's got a 13-11 record as a starter.

"It's a blessing to have a quarterback in your room who's started [24] games," Cousins said. "That's not very common. He speaks from experience, from having been there — from having succeeded and having failed — and that means a great deal to me. Every day in the meeting room and on the sideline during games, he can speak from a place of experience."

For Siemian, who began his career behind Manning and Brock Osweiler in Denver, his first season in Minnesota will be a new experience. He's no longer the developmental passer he was on the Broncos' 2015 championship team, but he's no longer in the running for a starting job. The pecking order in Minnesota is clear: Cousins is the starter, and Siemian will be asked to embrace the life of a backup, always one play away but rarely afforded more than cursory snaps in practice during the regular season.

"We're learning together and everybody's growing together, but certainly, the role's defined a little bit," Siemian said. "I'm leaning on Kirk as much as I can, learning from him. … As a [No.] 2, you're one play away, so you're taking your prep just as if you're the [No.] 1. For me, prep is virtually the same; I just don't get the reps."

Siemian, like Cousins, is learning a new offense under coordinator John DeFilippo, though both quarterbacks have experience in West Coast offenses similar to the scheme DeFilippo will run. Siemian has tried to squirrel away extra work wherever he can, studying film and slipping on a virtual reality headset to view it from a first-person perspective.

"It's been great for me the couple times I've used it. I'm still feeling my way through it," Siemian said. "Even Kirk, as a starter, will probably tell you he's not getting enough reps."

The nature of the NFL's offseason program, and the fact the Vikings have two new quarterbacks as well as a new coordinator, means all of the team's quarterbacks are trying to cram an extensive amount of work into 13 full-team practices. Cousins compared it to "drinking through a fire hose," and said he'll use much of the break between minicamp and training camp to revisit the offense in more detail.

DeFilippo said the Vikings are working with Siemian to speed things up somewhat, but added that's to be expected at this point of the offseason.

"It's not Trevor; it's just learning a new offense, getting acclimated, hearing the plays for the first time, all those things," DeFilippo said. "It's gotten better as we've gone along. We're always working on that with all of our quarterbacks, and I'm always working on myself."

Even as the Vikings hope for Cousins — who's started all 16 games the past three seasons — to continue his stretch of durability, they know as well as anyone they might need to call on a capable backup.

Shari L. Gross
Video (04:00) Vikings quarterback Kirk Cousins showed flashes of brilliance and threw out a new catchphrase at practice

The guy who knows it best might be DeFilippo, who as Eagles quarterbacks coach saw Nick Foles win a Super Bowl in relief of the injured Carson Wentz four months ago. And even if Siemian isn't the starter, the Vikings take great comfort in believing he could be if they need him.

"If you're a guy of Trevor's character, I think it's pretty easy [to accept the backup role,]" DeFilippo said. "He's all-in on supporting Kirk. If it was a player not of that character, it might become an issue. As a coaching staff, it's very settling for us that we have a backup who's started games and won games in this league."