Change was guaranteed as soon as Mike Zimmer became the Minnesota Vikings head coach and altered the team’s defensive approach.

The change was dramatic as Minnesota improved from one of the worst defensive teams in the league in 2013 to 14th last year in yards allowed and 11th in points allowed. Zimmer’s impact was immediate.

The alteration started in free agency last year. The Vikings signed nose tackle Linval Joseph on the first day of free agency and added cornerback Captain Munnerlyn a day later. The following week, Minnesota signed defensive tackle Tom Johnson.

And then Zimmer started coaching.

"I think you saw pretty drastic improvements in our defense from what you saw the year before under Zim’s system and how him and his coaching staff handled our players," general manager Rick Spielman said at the NFL Scouting Combine. "We feel very strongly about the core of young players, talented young players that we have. You saw the development of those guys and what our staff was able to do with Xavier Rhodes, with Sharrif Floyd, how Harrison Smith is. Even Anthony Barr, who we think is going to be a very unique talent. Now if we can continue to add to those pieces that we have built in already, we’re very excited about the direction of this team."

Here’s where the Vikings might be looking to add for Zimmer’s second season when free agency opens March 10:

1. Middle linebacker

The situation: Minnesota has seemingly been trying to fill the middle linebacker for a few seasons and looking for a true three-down linebacker who can stay on the field in nickel situations and succeed in pass coverage. E.J. Henderson at the end of his career was more of a downhill run stuffer, as was Jasper Brinkley in his return to the Vikings last season.

Brinkley held down the position in 2014, finishing fifth on the team with 74 tackles. He stopped the run, as expected, but was a liability at times in coverage. With Brinkley being a pending unrestricted free agent, Minnesota would surely like to find a younger option to settle in the position and be as adept in coverage as in run defense.

The incumbent: Brinkley perhaps re-established some of his value last season after it slipped in his one season in Arizona. Yet, Brinkley likely won’t be looking at breaking the bank in free agency, either. Without Brinkley, the player on the roster in the likely lead for the spot would be Audie Cole.

Cole again flashed in limited time last year, and even started at outside linebacker. Michael Mauti has been a middle linebacker, but Zimmer and the new staff seemed to feel he was a better fit outside. In the search for an answer, the Vikings likely will look to the draft as well.

The outside help: If Minnesota prefers a veteran option instead of relying solely on the draft, free agency will offer a few intriguing options.

Buffalo’s Brandon Spikes is similar to Brinkley in being a strong run stopper but having limitations in coverage. Spikes is younger (27) and a bit more dynamic. The Bills are said to be interested in re-signing Spikes. When Spielman steps out in free agency, he’s typically looking for younger players: under 30 years old and usually coming off their first NFL contract. Tampa Bay’s Mason Foster will be 27 when next season begins and could be an option. Tennessee’s Colin McCarthy, 26, has dealt with injuries but showed some talent when on the field.

Two names stick out, in particular. Cincinnati’s Rey Maualuga, 28, anchored Zimmer’s defenses with the Bengals in the middle. Maualuga is also known more for run defense than coverage. Cincinnati reportedly hopes to re-sign Maualuga, too. UPDATE: The Bengals re-signed Maualuga to a three-year contract Thursday.

The connection between Green Bay and Minnesota is becoming cliche, but Spielman has been known for signing former Packers. Green Bay released A.J. Hawk, 31, who played inside in the Packers’ 3-4 defense but likely could fit in the middle of a 4-3 defense, as well.

2. Safety

The situation: The Vikings have one certainty in Smith, a budding star and future Pro Bowl-level talent. Finding a partner for Smith is the issue. Jamarca Sanford had held down the spot for three seasons but was released before last season started.

Robert Blanton took over as a starter but eventually was replaced by Andrew Sendejo. Antone Exum was held to special teams duty in his rookie season but has the coverage ability Zimmer prefers from his safeties.

The draft doesn’t have many top options, outside of Alabama’s Landon Collins, who is believed to be more of an in-the-box safety. Minnesota might have to look to free agency if it doesn’t see a starter among Sendejo, Blanton and Exum.

The incumbent: Sendejo is the incumbent based on starting at the end of last season. Losing his job to Sendejo might signal Blanton’s lack of potential in this defense. Exum possibly could be a long-term answer if the Vikings and Zimmer see the potential and believe he can develop.

Sendejo has started 13 games over the past two seasons, but he’s still considered more of a special teams player. He’s a standout on special teams. He’s athletic and willing to hit. He had 27 tackles last season without an interception in 16 games and three starts.

The outside help: Minnesota isn’t the only team looking for a safety and with few options in the draft, free agents likely will end up cashing in. New England’s Devin McCourty is a talented player and has the ability to be strong in coverage. But McCourty is considered the top safety on the free-agent board and won’t come cheaply.

Denver’s Rahim Moore, 25, is best known for one play — coming up short on a long touchdown in a playoff loss to Baltimore three years ago — but he’s played well to get past the play and led the Broncos with four interceptions last year. Oakland released Tyvon Branch, 28, in a salary-cap related move. Branch has dealt with injuries but has talent.

Miami’s Louis Delmas, the New York Giants’ Stevie Brown, Buffalo’s Da’Norris Searcy, Philadelphia’s Nate Allen, Baltimore’s Darian Stewart and San Diego’s Marcus Gilchrist are some of the under-30 starters in free agency.

3. Cornerback

The situation: The thing about cliches is there is truth in them. The adage "you can never have enough cornerbacks" has been proven true by Minnesota over the years. The Vikings benefitted from health at the position last year but had seen injuries ruin the position for several years previously.

Healthy, Minnesota’s cornerbacks made a dramatic improvement last season. Xavier Rhodes took a big step and looks to have the potential to be one of the league’s top cornerbacks. Josh Robinson recovered from a poor 2013 season to be steady on the outside. Munnerlyn didn’t live up to being a top free-agent pickup but he’s been an effective starter for years and should be expected to bounce back in his second season with Zimmer.

Increasing depth should be the objective for the Vikings this offseason. Minnesota can feel good about the progress of Rhodes and Robinson and also the potential of Jabari Price off his rookie season. But — the cliche again — they can never have enough cornerbacks.

The incumbent: Rhodes could be looking at a possible perennial Pro Bowl-type career. The second half of the season, Rhodes became a true shutdown cornerback while facing some of the league’s top receivers. Robinson had a strong season but could possibly slide down the depth chart if the Vikings can add a top-level starter. Munnerlyn is best playing in the slot though he was a starter on the outside in the base defense last year.

Price showed some ability in his rookie season in limited time. Marcus Sherels has held up when asked to play cornerback but makes his roster spot based on punt returning.

The outside help: Minnesota could look at a potential starter in free agency and have Robinson slide down. There are several intriguing names among the free agents.

Byron Maxwell will likely be the most sought-after free agent. Pilfering Seattle’s cornerbacks has been popular and Maxwell is the next in line. He is 27 and will likely draw significant interest in free agency. The Green Bay connection comes in the form of Tramon Williams and Davon House. Williams will be 32 when next season begins. House, 26 in July, is steady and has been behind Williams and Sam Shields with the Packers.

Some of the other top-tier options include San Diego’s Brandon Flowers, Houston’s Kareem Jackson and Cleveland’s Buster Skrine. Others with potential to grow with Zimmer’s coaching are San Francisco’s Perrish Cox and Chris Culliver, New Orleans’ Patrick Robinson, New York Jets former first-round draft pick Kyle Wilson, Jacksonville’s Will Blackmon and Walter Thurmond from the New York Giants.

A Zimmer student, while older, is Cincinnati’s Terrence Newman. Newman, 36, played for Zimmer in Dallas as a first-round draft pick and later joined him with the Bengals. Would Newman follow Zimmer again?

4. Backup defensive line

The situation: Zimmer prefers a rotation among his defensive linemen and it was certainly the case last season. Minnesota dipped into free agency last year for two veterans able to rotate with the young line the Vikings had developed.

One of last year’s free agents, Johnson, increased his value as he’s set to hit free agency again. Johnson was second on the team and set a career-high with 6.5 sacks. Corey Wootton was a steady player but didn’t provide a big impact in backing up Everson Griffen and Brian Robison at end. Wootton is a free agent again too.

The incumbent: Minnesota had the two veterans and two rookies as the backups along the line last year, with Justin Trattou seeing limited time. Along with Johnson and Wootton, rookies Shamar Stephen and Scott Crichton were the top backups.

Stephen played a big role as a backup defensive tackle after being a seventh-round draft pick. He can logically be expected to pick up more time, if needed. Crichton didn’t see much time at defensive end, but he has pass-rushing skills as a third-round draft pick. Perhaps the Vikings will be comfortable in expecting more development from Stephen and Crichton to be the rotational backups.

Crichton could step into Wootton’s spot as a backup end. Zimmer probably would like another rotational tackle to team with Stephen behind Joseph and Sharrif Floyd. Johnson was a good fit in Zimmer’s system as the under tackle. He won’t be the bargain he was last year, but Minnesota could look to bring Johnson back. Stephen can play some 3-technique but was the main backup to Joseph at nose tackle last year.

The outside help: Johnson would likely be the top option and the Vikings wouldn’t have to look outside. But if Johnson capitalizes on his strong season and signs elsewhere, Minnesota could hope for more from Stephen and Crichton and perhaps draft more depth.

Behind players who likely will be looking for starting jobs, the Vikings will probably wait out for some bargains like they landed with Johnson and Wootton last year.

Inside, Tennessee’s Karl Klug is a Johnson-type signing. Klug, 26, is a Minnesota native. Kevin Williams is a free agent again after one year in Seattle. Baltimore released Chris Canty, 32, who played for Zimmer in Dallas.

There are plenty of bigger names as defensive ends. Deciding which have to settle for backup jobs will be interesting to watch. Some of the 4-3 ends have spent time as 3-4 outside linebackers. Most have been considered more as pass rushers than ends able to set the edge against the run or pass. Philadelphia’s Brandon Graham should see considerable attention after being a part-time player for the Eagles.

Tampa Bay’s Adrian Clayborn, Dallas’ George Selvie, Seattle’s O’Brien Schofield, Atlanta’s Kroy Biermann and Tampa Bay’s Larry English are some of the under-30 options who could fit as a 4-3 defensive end.

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