The Vikings history book is filled with starting quarterback controversies, and the latest involving Case Keenum and Kirk Cousins might beat them all.

The speculation is that Cousins will sign Thursday after coming to Minnesota to meet with the Vikings at the start of free agency, which opens at 3 p.m. Wednesday.

Cousins, the top free-agent quarterback available, is expected to sign a three-year deal worth a guaranteed $84 million and be a Viking by the end of the week.

From trusted sources and common sense, I will try and put together the story of how I believe the Vikings made the decision to pursue Cousins over Keenum, Teddy Bridgewater and Sam Bradford.

The Vikings quit negotiating with Bridgewater, who reached an agreement with the Jets on Tuesday, when he demanded a contract over $10 million.

They never negotiated with Bradford because there was no sure bet his knees could handle another season. The word is he will sign with the Cardinals for $20 million, with $15 million guaranteed.

Until recently, the Vikings considered re-signing Keenum. But when Keenum started looking for a contract that would pay him around $20 million annually, the Vikings became even more interested in Cousins.

And on Tuesday, when word leaked that Keenum was headed to the Broncos, Cousins coming to the Vikings became all but guaranteed.

If Cousins, who has so much experience, wasn’t available, the Purple probably would have signed Keenum, even at a high cost. But Vikings officials, while pleased with Keenum’s performance last season, weren’t certain he could repeat it.

Keenum had an incredible year. He stepped in for the injured Bradford when the Vikings were desperate and proceeded to go 11-3 while throwing for 3,547 yards, 22 touchdowns and only seven interceptions. He completed 67.6 percent of his passes and was sacked only 22 times, partly because of his mobility.

Cousins passed for similar numbers last season, throwing for 4,093 yards, 27 TDs and 13 picks, and completed 64.3 percent of his passes. But when the Vikings staff started evaluating film of Keenum and Cousins, there was no comparison.

Superior system

The two quarterbacks’ statistics were similar last season, but Cousins has a track record of performing at a high level for several years. And while Keenum was helped by one of the best defenses and running games in football, Cousins was not.

The Vikings defense ranked first in both total defense (275.9 yards allowed per game) and scoring defense (15.8 points allowed), the Redskins ranked 21st in total defense (347.9) and tied for 27th in scoring defense (24.3).

The Vikings also had the seventh-best rushing offense in the NFL, averaging 122.3 yards per game. The Redskins ranked 28th at 90.5 yards per game.

There is no question Keenum benefited from the Vikings’ overall play, while Cousins had to carry the Redskins for much of the past three seasons. During those three years, Cousins averaged 4,392 yards and 27 touchdowns per season while completing 67 percent of his passes over 48 starts. He is the only NFL quarterback to average at least 4,000 yards, 25 touchdowns and a 90.0 or better quarterback rating over the past three seasons.

Over that same stretch, Keenum averaged 2,192 yards and 12 touchdowns per season while completing 64.3 percent of his passes.

Yes, Keenum had one stellar season but he didn’t have the proven longevity of Cousins. That and his contract demands moved the Vikings toward Cousins.

A long-term solution

Cousins would be the sixth opening-day starting quarterback for the Vikings in six seasons.

Last season it was Bradford who started Week 1. Before that it was Shaun Hill (2016), Bridgewater (2015), Matt Cassel (2014) and Christian Ponder (2013).

The Vikings have had some of the worst luck in the NFL when it comes to quarterbacks.

With Cousins, they know he has shown the ability to stay on the field over several seasons. He hasn’t missed a start in the past three years.

The last Vikings quarterback to start 10 games or more over three consecutive seasons was Daunte Culpepper over a decade ago.

While some are concerned over the size of Cousins’ contract, consider he was making $24 million with the Redskins last season. And earlier in the offseason, the 49ers gave Jimmy Garoppolo a five-year deal worth $137.5 million with $90 million guaranteed over the first three seasons.

Garoppolo has made seven career starts.

This is the going rate for an NFL quarterback, especially a quality starter.

The Vikings needed to find a quarterback they believed could not only help them for one season, as Bradford, Bridgewater and Keenum did, but could do it for a long period of time while staying healthy. They have to hope Cousins is that guy.

Super Bowl or bust?

Can Cousins take the Vikings to the Super Bowl? Owners Zygi and Mark Wilf must think so if they’re planning to guarantee the former Michigan State quarterback $84 million over three years.

The pressure will be on Cousins to produce, because the Vikings reached the NFC Championship Game last season and should have an equal or better roster heading into the 2018 schedule. They needed to get Cousins, because next year’s schedule is really difficult. The Vikings’ road schedule most likely will be the toughest in the NFL, including games against the Rams, Packers, Seahawks, Patriots and Eagles.

Still, the Vikings could be second-guessed if any of the following happens: Keenum goes to Denver and thrives as a full-time starter; Bradford goes to the Cardinals and proves he’s healthy and plays like he did in 2016; or Bridgewater proves his knee is fully healed and he becomes a winning quarterback with the Jets.

But for now, the Vikings made the best move available to them and signed the top quarterback on the free-agent market, who could solidify that position for years to come.

Sid Hartman can be heard on WCCO AM-830 at 8:40 a.m. Monday and Friday, 2 p.m. Friday and 10:30 a.m. Sunday. shartman@startribune.com