Lynne Sladky/Associated Press

The roller-coaster career of Minnesota Vikings left tackle Matt Kalil is entering its most important stretch, and the former No. 4 overall pick is now doing everything he can to make sure his body cooperates for the ride.

According to Tom Pelissero of USA Today, Kalil underwent surgery on both of his knees back in January. His rehabilitation has included specialized injections for joint pain and platelet-rich plasma treatments to the problem areas. The underlying hope is that Kalil's ongoing issues with lower-body injuries will finally dissipate, and Minnesota's Pro Bowl left tackle will return to form for his vastly important 2015 season.

Kalil hasn't missed a game in three seasons, but problems with his knees have been at least part of the reason for his incredible regression since 2012.

After looking like the NFL's next big thing at left tackle as a rookie, Kalil has seen his career enter an unforeseen free fall—with last season representing rock bottom.

According to Pro Football Focus, Kalil allowed more sacks (12) than any other tackle in football in 2014. Only rookie Ja'Wuan James of the Miami Dolphins gave up more total pressures than Kalil's 53, and his final grade at PFF (-29.1) ranked 81st out of 84 qualifying tackles. Needless to say, Kalil was one of the worst tackles in the NFL last season.

For comparison's sake, consider that Kalil was the 21st-best overall tackle and No. 1 rookie tackle in 2012. He was selected to the Pro Bowl. His discouraging slide began in 2013, when he allowed 12 quarterback hits and 49 total disruptions as the site's 51st overall tackle.

Yet 2013 was also the same season his knees started acting up. Talent can quickly be overtaken by physical ailment. The effect seems even more pronounced in Kalil's case.

Per Pelissero, Kalil dealt with at least one knee injury requiring surgery during his time at USC. He's now had arthroscopic surgery on his knees in back-to-back offseasons in the NFL. That's a lot of surgical work done in a short period of time for a position so dependent on lateral mobility and bending at the knees.

The Vikings still picked up Kalil's fifth-year option last week, as first reported by Alex Marvez of Fox Sports. The move was expected, given the fact that the option year is guaranteed against injury only.

Bruce Kluckhohn-USA TODAY Sports

Matt Kalil: Regression of a Left Tackle PFF Grade Sacks Allowed QB Disruptions Allowed 2012 +12.1 2 23 2013 -6.0 4 49 2014 -29.1 12 53 Source: Pro Football Focus

Translation: If Kalil struggles again in 2015, the Vikings can cut him before the start of the 2016 season with no salary-cap penalty (assuming no major injury occurs). If he plays well, Minnesota will have its starting left tackle under contract through 2016 at a manageable one-year price. It was a no-brainer decision for general manager Rick Spielman.

The onus is now on Kalil to determine where he plays beyond next season.

Head coach Mike Zimmer is confident the early-offseason surgeries will help his left tackle rebound in 2015.

“Matt’s told me that this is the best he’s felt since his rookie year, I think,” Zimmer said, via Matt Vensel of the Minneapolis Star Tribune. “So we’re just going with that. He said he feels great. And, you know, we’re glad that we got the issues taken care of.”

As a rookie, Kalil played more than 1,000 snaps and allowed only 23 total disruptions, including just two sacks. He finished the season ranked sixth among offensive tackles in PFF's pass-blocking efficiency. He was also part of an offensive line that paved the way for Adrian Peterson's 2,000-yard rushing season.

Two short years later, the Vikings may already be grooming potential replacements.

Last week's NFL draft did not see Minnesota use a high pick on an offensive tackle. However, Spielman did use two later-round picks at the premium position. Pitt's T.J. Clemmings was selected in the fourth round, while Oklahoma's Tyrus Thompson was taken two rounds later in the sixth. Both have the athleticism and body type to play left tackle in the NFL. If one of the two takes a significant jump as a rookie—neither currently looks like an immediate starter at left or right tackle—it could make it easier on the Vikings to dump Kalil and insert the cheaper young player on the left side in 2016.

The situation paints a clear enough picture for Kalil. Either he stays healthy and plays well in 2015, or he'll likely leave Minnesota as a potential bust before the 2016 season.

Luckily for Kalil, he should enter next season the healthiest he's been since 2012. He took care of his knee issues early in this offseason (as opposed to last year, when it cut into his training camp preparation), which should give him ample time to get to as close to 100 percent before August as possible. Coming to camp healthy could give him the mental edge he's been so clearly missing the past two seasons, when bad plays seemed to compound during games.

"I think this year he’s planned (the surgery) out pretty well," Matt's dad, Frank Kalil, told Chris Tomasson of the St. Paul Pioneer Press. "He’s participated in all the offseason workouts right now, and he looks good. I think that he’ll have a great season."

A great season from Kalil could revive a flat-lining career. Even some evidence of improvement would likely be enough for the Vikings to keep Kalil past 2015. But with the way his play is currently trending, nothing is guaranteed.

First and foremost, the Vikings need their left tackle healthy. Most everyone around Kalil believes his regression is directly tied to his recent knee injuries. With those issues taken care of early in the process this time around, Kalil has been provided an opportunity to turn his free fall around and secure his place with the Vikings past next season.

Zach Kruse covers the NFL for Bleacher Report.

Follow @zachkruse2