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In every draft, there are a number of talented players who end up in the "wrong" system—one that hides their strengths and emphasizes their weaknesses. Each team has a scheme that will take advantage of a different set of talents and the Vikings are no different. In that way, they are restricted in the pool of available players who can benefit the team.
But sometimes, players are so talented that it would be a waste not to select them, regardless of scheme. The balance between talent and scheme fit is one that teams try to find the better side of every year. Players find themselves in poor schematic fits nearly every year, and the talent that teams saw sometimes goes away.
This happens in free agency often, where a player who was talented in one system disappears in another—Nnamdi Asomugha is an excellent example, where an exclusively man-free corner who played on the right ended up playing zone on either side with the Eagles—and lost nearly all of his acclaim. Glenn Dorsey is an example of a player who was drafted for the wrong scheme and seemingly disappeared. Dorsey was a 3-technique defensive tackle for a 4-3 system but was forced to play defensive end in a 3-4 system.
The easiest solution is to "design your scheme around available talent," but that's easier said than done. The perceived flexibility of Bill Belichick has increased the popularity of this suggestion, but it doesn't make it a simple fix. Belichick often found players who fit his schemes perfectly, but were deficient at a number of other things that made them cheap—which is not quite what proponents of the "talent-first" approach are advocating.
The issue with finding talent first and scheme second is fairly simple—scheme philosophies are dependent upon each other for success. There are cascading effects to a scheme change to accomodate a single talented player that places different demands on other players on the field—one they may not be able to meet. For example, if the Vikings wanted to eschew finding a nose tackle in favor of a very good pass-rushing under tackle, they would be forced to implement a "Wide 9" defensive front, leaving both "A" gaps (the gap on either side of the center) open.
This system does a good job of creating pressure from four linemen, but it creates additional stress on the linebackers, who are fully responsible for the run game. With three linebackers responsible for four or even five gaps against pro sets, it's difficult to stop the run without top-tier linebackers who can read blocks as well as plays.
So, drafting one player and changing the scheme to put him on the field changes the requirements for nearly every other player on the field. It is not simple or useful to change the scheme in response to draft talent available, because it could downgrade the on-field production or scheme talent of the players on the roster at the moment. Rare examples, like Bill Arnsparger's initial 3-4 at Miami or Bill Parcell's "true" 3-4 front at New York have captured imagination, but failed examples (like the Vikings' brief experiment with the 3-4) have been forgotten.
Even the perceived master of scheme flexibility, Bill Belichick, has found problems adjusting his talent and scheme to match. The transition to a 4-3 had nearly neutralized Vince Wilfork and made him a liability for most of 2012. The same thing happened to Devin McCourty his sophomore year after a phenomenal rookie season because Belichick switched to a zone system to accommodate players. McCourty was targeted again and again.
So, when the Vikings draft, they have to be very sensitive to scheme. Without it, they could end up with another Jasper Brinkley.
This means when face with a choice between a man corner and a zone corner—say Xavier Rhodes or Desmond Trufant—they'll want to bias their decision towards the zone corner. Same with middle linebackers, where deep drops and zone coverage is important. If the Vikings draft a guard, they need to find someone with extremely good footwork above all else, given the complication of the Vikings' run game.
If the Vikings find ways to draft talent and scheme fit, they'll have set themselves up for a very long time.