Ann Heisenfelt/Associated Press

On the Minnesota Vikings' to-do list this offseason, re-signing free agents is a pretty low priority. The future of star running back Adrian Peterson will be the biggest story, and what happens with him will have a big impact on what the Vikings do in both free agency and in the draft.

As far as their own free agents go, the Vikings really have no pressing concerns.

Middle linebacker Jasper Brinkley is the only starter in the group, and his value decreases because he gets off the field in Minnesota's sub-packages.

This isn't to say, however, that Minnesota has no work to do in keeping its own players. Of all the Vikings free agents, defensive tackle Tom Johnson had the biggest impact in 2014 and is the closest thing Minnesota has to a "must retain" player.

The Vikings certainly got their money's worth with Johnson in 2014, signing him to a one-year deal for $845,000 last offseason. Just a rotational player, Johnson, who backed up defensive tackles Sharrif Floyd and Linval Joseph, finished second on the team with 6.5 sacks and also had 22 tackles and a forced fumble.

Of 72 qualifying defensive tackles, Pro Football Focus ranked Johnson 12th in terms of pass-rushing efficiency. According to PFF, Johnson played just 447 snaps compared to starter Linval Joseph's 744, which makes his sack total all the more impressive.

A look at some of the top free-agent defensive tackles on the market proves what a bargain Johnson was for the Vikings in 2014:

Player Team 2014 Salary Tackles Sacks Ndamukong Suh Detroit $22.4 million 53 8.5 C.J. Mosley Detroit $1.54 million 26 2.5 Terrance Knighton Denver $2.75 million 30 2 Dan Williams Arizona $2.15 million 32 1 Jared Odrick Miami $2.77 million 29 1 Pat Sims Oakland $2.0 million 25 0 Ahtyba Rubin Cleveland $8.17 million 28 1 Dwan Edwards Carolina $2.01 million 40 4 Tom Johnson Minnesota $845,000 22 6.5

Of those listed, only Sims and Edwards played backup roles like Johnson.

Obviously, sack and tackle numbers don't come close to telling the whole story of how effective defensive tackles are, but Johnson's sack productivity in 2014 has earned him a pay raise in the coming seasons.

Johnson has certainly paid his dues as a football journeyman, playing for six different teams over the last nine years, including stops in the Arena Football League and the CFL. Johnson probably hopes he's done moving around, and as Matt Vensel of the Star Tribune reports, he's looking for a multiyear deal.

Johnson was one of the more valuable reserves on the Vikings' roster in 2014, as Sharrif Floyd battled a nagging knee injury for most of the season.

Johnson and Floyd combined for 11 sacks on the season, and while the hope is that Floyd continues to improve in his third season, having a veteran backup like Johnson will only help him develop.

Though it surely must irk defensive-minded head coach Mike Zimmer that his run defense ranked 25th in the league in 2014, the team seems to have good depth at tackle with Linval Joseph and rookie Shamar Stephen both showing good glimpses during the season.

Joseph played decently but didn't come close to paying off on the five-year, $31.25 million deal the Vikings signed him to last offseason.

Minnesota has to hope that another year under Zimmer will help Joseph, as it has with players in the past.

Cincinnati Bengals tackle Geno Atkins, playing in his third Pro Bowl in four years, gave Zimmer a lot of credit for his success in the league, according to Mike Tomasson of the Pioneer Press:

It was a pleasure to have Zimmer as a coach. I learned a lot from him. He helped my game get to the next level and I’m very thankful for that. He helped me in understanding the game, what the offense is trying to do and he always did a great job with putting players in position to make plays and just making sure everybody did their job while focusing on the team and not individual play.

Now, the Vikings certainly don't want to overpay for a guy who will be 31 years old when the 2015 season begins and had never recorded more than two sacks in a season before 2014.

Wilfredo Lee/Associated Press

Johnson deserves a raise, but it's unlikely that Minnesota would want to pay him much more than $6 million over a two- or three-year deal.

As we said at the top, the Vikings have themselves in terrific financial shape as they head toward free agency. They took care of their only two really meaningful 2015 prospective free agents last summer when they signed both tight end Kyle Rudolph and guard Brandon Fusco to extensions.

Depending on what happens with Peterson, the Vikings might be big players in free agency this year, but locking up Johnson before he hits the open market would serve them well.

Contract information courtesy of Spotrac.