Case Keenum is playing the best football of his career, and that probably means he’s not going to stay a Viking for very long.

Ranking eighth in adjusted net yards per attempt, 14th in passer rating and third in ESPN’s Total QBR, Keenum’s passing is miles ahead of where it was last season — 28th, 27th and 30th in those respective statistics— or his career, where he ranks 30th and 32nd in ANYA and passer rating of 33 quarterbacks with 1,000 attempts.

At the moment, Keenum seems to be playing himself into a starting job somewhere, and if the Vikings are as confident in Teddy Bridgewater as they appear to be, it will be difficult to re-sign him and Keenum at the same time.

Much has been made of rising quarterback values for otherwise mid-tier players — and we’ve noted those as well — and nothing exemplifies that better than Mike Glennon’s contract with the Chicago Bears, something that pops up in nearly every discussion about quarterback contracts.

There’s not much that actually connects Glennon’s situation with Keenum’s. Glennon had been effectively benched three times throughout his tenure in Tampa Bay. After starting 13 games in 2013 for the Tampa Bay Buccaneers, the team signed Josh McCown to start over Glennon.

Glennon started five games in 2014 when McCown injured his thumb, but was benched swiftly afterward. In 2015, the Buccaneers let McCown hit free agency, but instead of giving Glennon free reign over the offense, the team opted to draft Jameis Winston first overall.

The North Carolina State product never performed all that well aside from a few statistical quirks, with a respectable touchdown-to-interception ratio in 2013 (19:9) but not much else — he ranked 29th in adjusted net yards per attempt, 38th (!) in net yards per attempt and 37th in yards per attempt.

Despite all that, the Chicago Bears invested $45 million over three years in Glennon, even though he hadn’t played meaningful football for three seasons when they signed him.

Glennon signed his contract with the Bears at 27 years of age and could have been conceivably pitched as a potential franchise quarterback. He didn’t have too many passing attempts (630) or starting years (one) under his belt, which means he didn’t have much time to put many bad throws on film — so those that liked him coming out of college can project his best qualities on him without having to overcome years of bad play to tamp it down.

Keenum, on the other hand, has had 1,039 passing attempts — and could finish the year with over 1,300 — and three years where he’s started for most of the season. There’s less room for error, and he’s had more than one down season.

When negotiating his next contract, he’ll be 30 years old and already past his “prime” — a term that has lost some meaning with players like Peyton Manning, Tom Brady and Drew Brees pushing the theoretical maximum age for quarterbacks, but still carries weight in negotiations.

But there are a few quarterbacks who have had unusual peak seasons in a contract year, and we can compare those rare quarterback moments to the Keenum situation.

The most memorable examples of outlier quarterback play both come from 2013, where Nick Foles and McCown ranked first and third in adjusted net yards per attempt. McCown was in a contract year, and plied his way to replacing Glennon after the season, but not for very much money despite being signed as a starter — 3.8 percent of that year’s salary cap, $6.3 million in 2017 money.

On the other hand, Foles was traded shortly after his remarkable season and signed a new deal nearly immediately for 6.1 percent of that year’s salary cap — $10.2 million in 2017 money.

The problem with the McCown comparison is that he already had 11 years of NFL experience under his belt and signed that deal with Tampa Bay at age 35. Foles, on the other hand, had three years of NFL experience and signed a new deal at 26.

It might be tempting to split the difference to predict Keenum’s contract, but it’s always good to get more data to inform our decision before projecting what his contract might be.

There are a few quarterbacks in the past 10 years who have had unusual peak seasons aside from McCown and Foles in 2013. Matt Cassel took over for Tom Brady in 2008 and was an unknown seventh-round draft pick who backed up Heisman Trophy winners Carson Palmer and Matt Leinart without starting a game in college.

Before 2008, most people only knew him as Tom Brady’s backup, beating out players like Doug Flutie, Vinny Testaverde and third-round pick Kevin O’Connell for the second spot on the roster. After a stellar 2008, he was traded to Kansas City and in the offseason signed a six-year deal for $62.7 million — an average that accounted for 8.5 percent of 2009’s salary cap.

Like Keenum, one could fairly diminish Cassel’s overall performance with discussions about his supporting cast. With Wes Welker and Randy Moss, Cassel had the best receiver corps in the league. Not only that, Cassel had a fantastic offensive line with Matt Light, Logan Mankins and Dan Koppen leading the way.

Vince Young and Josh Freeman both had atypical years in 2010, but only Young hit free agency in the year following. Young’s case is unusual because his best year was followed by an injury, an altercation with a head coach and a release from the team. He then signed as a backup for the Philadelphia Eagles on a one-year, $4 million deal.

Speaking of the Eagles, Michael Vick’s 2010 allowed him to grab the fourth-largest quarterback contract in the NFL the following year, worth $16 million, or 13.3 percent of that year’s salary cap.

After that, Ryan Fitzpatrick produced an outlier six-game stretch, where he generated a passer rating of 95.3 — compared to a career rating of 73 up until that point. He signed a new deal in the middle of his age-29 season (2011) after throwing 1,379 passing attempts worth $59 million over six years, or 8.2 percent of available cap space that year.

Weirdly, Fitzpatrick had another outlier year in 2014 for the Houston Texans, once again producing a passer rating of 95.3 — this time over the course of 12 games instead of six — and signed with the Jets in free agency at age 33 for 8.4 percent of that year’s cap, $12 million.

Before Fitzpatrick was with the Texans, Matt Schaub had earned two Pro Bowl appearances as the primary passer for Houston. One was in 2009 and the other in 2012. 2011, however, was his best year statistically and he turned that year into a new contract one week into the 2012 season, signing for $15.5 million a year, or 12.9 percent of that year’s cap.

Schaub had one Pro Bowl, eight seasons and 2,279 pass attempts under his belt before signing a four-year extension with the Texans.

Colin Kaepernick in 2012 produced a year that we later recognized as an outlier for his overall performance, and though he earned a new deal in 2014 worth $19 million per year, it’s pretty easy to argue that his case doesn’t set precedent because of how early his peak season was in his career (age 25) and the fact that the team went to the Super Bowl.

Nevertheless, Kaepernick’s 14.3 percent cap charge does provide another data point to examine.

The only other surprising quarterback performances would include Drew Stanton’s 2014, which wasn’t impressive more than it was just substantially better than his other work. That earned him an Arizona Cardinals contract worth 2.1 percent of the cap.

Palmer, a fellow Cardinal and former Cassel teammate as well, had a year that defied expectations in 2015, leading the league in adjusted net yards per attempt. That gave his agent the opportunity to negotiate a contract worth $21 million a year on average, or 13.5 percent of the cap.

Like with Kaepernick, this isn’t a great parallel. Palmer had previously been to several Pro Bowls and threw the ball 5,443 times before signing a new deal.

The only other recent instance of an outlier year creating a new deal for a quarterback is Brock Osweiler, who signed a deal worth 11.6 percent of the cap, or $18 million per year. Again, a young player (26 at time of signing) without many prior attempts (305), Osweiler had more upside and time to convince a team that he could be the future.

The closest comparisons across age, prior pedigree and overall performance are probably Fitzpatrick in 2011. Generally, quarterbacks who sign a new deal while around 29 years old with a good number — but not too many — of attempts under their belt will sign it for about 9.3 percent of the salary cap.

In 2017, that would be $15.5 million. In 2018, according to Over the Cap, it could be $16.6 million.

It is important, however, to recognize that quarterback contracts in 2016 and 2017 are markedly different than quarterback contracts in 2011.

Sixteen years ago, the average of quarterbacks in the top-five of cap liability made twice as much as the average of quarterbacks in the top-20. This year, they only make 20 percent more.

If one looks at the rank of quarterbacks in salary when they signed their new deal instead of percentages, 15th seems to be the median conclusion. That conclusion includes unreasonable comparisons, however, like players too young to compare (Osweiler and Cassel are good examples) or with too many previous accolades (like Pro Bowlers Vick, Schaub and Palmer).

Players too old, like McCown and Palmer, also present comparison problems.

With all of that in mind, the Fitzpatrick comparison may be the best guide, given similar age, experience, prior reputation, draft position, history with multiple teams and general team role (as a backup). He ranked 19th in salary.

Foles isn’t a bad comparison either, and he has a Rams connection to boot. Foles signed a new deal with three years of experience, a season of statistics that most people agreed did not credibly represent him and after bouncing around a few teams. He ranked 20th in quarterback salary with his new deal.

Cassel would also be a good guide, but there are unfortunately only very limited data to compare his salary to — most salary databases do not go back to 2009. A rough guess based on available data tells us his deal may have ranked around 14th in the NFL, and that could represent a decent upper-bound for Keenum’s next salary. Given the supporting cast and system arguments one would be tempted to make when discussing Cassel’s salary, Keenum seems like a fine comparator.

Assuming that the trend for mid-tier quarterback salaries flattens out and remains near 80 percent of a top-five contract, Keenum could earn a contract that averages anywhere between $17-$22 million.

It seems unlikely Keenum breaks $20 million, but new cap room does funny things. At the very least, Keenum is on track for a good paycheck and may get just as much as Glennon did in Chicago, $15 million.

On the other hand, Keenum will be competing with an unusually crowded quarterback market.

While it might not be fair to call the market for quarterbacks robust, teams looking for signal-callers to act as starting stand-ins while a rookie develops will certainly have their options. And there will definitely be teams who will have played themselves out of a prime draft spot that might take shots at finding a starter.

Assuming Brees re-signs in New Orleans and Jimmy Garoppolo re-signs in San Francisco, the highest-profile quarterbacks hitting the market will be Tyrod Taylor and Kirk Cousins. Both should earn starting salaries with a new team and could make the market more difficult for Keenum who would likely earn significantly less than either of them.

There are also veterans with more experience that teams may be more comfortable handing intermediate-term reins to, like McCown and Fitzpatrick.

A.J. McCarron is a young signal-caller who will be competing with Keenum for a coveted potential starting spot and he could command an interesting market.

Aside from him, two young players who have an age advantage over Keenum — though not necessarily a performance advantage — are Glennon and Blake Bortles. Bortles isn’t likely to command a huge market, but it also wouldn’t be extraordinarily shocking to see an NFL team fall in love with Bortles’ 2015 season and assume they can fix him.

That leaves the best of the bunch: Alex Smith and Sam Bradford. Though Smith has another year on his contract, the Chiefs may trade him to a contender for draft picks while relying on their first-round pick, Patrick Mahomes, to line up under center.

Both Smith and Bradford are game managers who blew up this year, but Smith’s availability is only theoretical while Bradford’s health is a significant liability.

Teams like the Denver Broncos, Jacksonville Jaguars, Buffalo Bills and the Washington Redskins could provide decent markets for quarterbacks because they may play themselves out of an important draft pick while also operating as franchises that are legitimately only a passer away from competing.

The New York Jets may, absurdly, also play themselves out of a draft pick while the Arizona Cardinals, Los Angeles Chargers and New York Giants could try to find their quarterback of the future.

Still, with at least five viable competitors and up to ten players jostling for a few quarterback spots, Keenum’s value is depressed. Glennon mostly competed against Brian Hoyer and McCown for money as Cousins and Taylor took themselves out of last year’s market a short time before free agency started. Foles did enter the free agency market but explicitly signed on as a backup and was more in Fitzpatrick’s market than Glennon’s.

Taking all of that into consideration, Keenum could still find himself making too much money for the Vikings to compete for his signature. Though typically, the market would pay nearly $18 million for the resume Keenum has put together, the impact that all of these quarterbacks will have in free agency is unknowable.

Even a depressed contract number could be too rich for the Vikings. Keenum’s price floor is well over $12 million, even after taking into account the market effects of a quarterback supply glut. In fact, his contract total has a strong possibility of going over $12 million, making him untenable as a signee with the Vikings.

With that in mind, the Vikings will need to find a backup on the market that they can trust. There are a few that will hit free agency.