5. Michael Vick

Howard Smith

In any offensive system, the quarterback has a chance to ruin it. Michael Vick is a good fit for a Kelly offense, but his career is on the downswing. Having a mobile quarterback for Kelly is not necessarily needed, but certainly a plus. Vick’s running will come in handy, as will his big throws downfield to DeSean Jackson.

Vick has had trouble with accuracy and fumbling, and almost any system in the NFL will fail with a QB that is not careful with the football. Further, especially in the NFL, a quick offense system relies on the quarterback to make very good decisions at the line, something Vick is not known for. Vick is no Peyton Manning at the line, and in the NFL, that matter a lot. At Oregon, quarterbacks could use simple plays over and over, and the defense would not disguise its plays nearly as well. In the NFL, reading and adjusting to a defense is imperative, and Vick may just not be up to the job.

4. Quarterbacks Get Injured too Often

Dale Zenine

Unfortunately, sights like this one have become common for Eagles fans, and they probably are not going anywhere with the Kelly system. In any system that uses quarterbacks for running, injuries are a major concern. The reason that plays like the spread option are only very slowly being incorporated into the NFL is that when a star quarterback gets hurt, the entire offense suffers greatly. It is great to have RGIII, but if he keeps getting injuries, his running skills are not viable in the NFL.

Kelly’s system can function without a lot of quarterback runs, and that will probably be one of the first changes he makes coming into the NFL. If he does not, Vick will continue to get injured as he always does, and the Eagles' season could turn ugly fast.

3. Maclin's Injury

Matthew Emmons-USA TODAY Sports

Wide receiver Jeremy Maclin getting hurt is no fault of Chip Kelly’s, but it could mean major issue for his system. When Maclin was put on injured reserve with an ACL tear, Kelly must have known it was a setback. In a Kelly offense, a solid pass catcher going over the middle and on short routes is necessary, but the Eagles really do not have that without Maclin. DeSean Jackson can run deep routes, but avoids going over the middle. Maclin would have been a perfect counterbalance and would have greatly helped spacing on the field. Without proper spacing, Chip Kelly’s offense simply cannot function, so the loss of Maclin could be serious.

2. Open-Field Tackling in the NFL is much Better than in College

Howard Smith-USA TODAY Sports

One of the principles of a spread offense is that it creates one-on -one situations across the field, putting a lot of pressure on secondary players who have trouble tackling. In college, this is unstoppable against many defenses, but in the NFL, players are simply better tacklers, making such situations less beneficial to NFL offenses.

A great way to illustrate this is the Stanford-Oregon game last season. Oregon ran 77 plays and only scored 14 points. Stanford has an NFL-style offense and defense with strong tacklers at every position. The Oregon offense was predicated on breaking tackles, so when Stanford could counter that, it made a big difference.

1. Player Isolation Does not Work as Well

John Geliebter-USA TODAY Sports

Chip Kelly’s offense goes even further than just getting into open field tackling situations, however. It’s about isolating defensive players and exploiting their mistakes. For example, Kelly’s inside zone read is predicated on letting a quarterback watch the end, figure out where he is weak, and capitalize on it. In the NFL, defensive ends will rarely be out of position. Not only that, but NFL defenses will mask their plays and bring complex blitzes to confuse any quarterback trying to run it. Isolating and exploiting defenders in the NFL is simply harder. This is part of the reason Kelly has already admitted that the zone read will become less key to his offense in the NFL.

Player isolation also happens on the passing end. Oregon racked up yards by simply targeting bad cornerbacks and throwing that way. In the NFL, there are few easily-beaten cornerbacks. When a cornerback is overmatched, NFL defenses have solutions, complex zones, or careful safety help. College systems are simpler, and even star safeties have more trouble giving help to cornerbacks.

When it is all said and done, Chip Kelly’s offense may work. He’s a smart guy. But so are the defensive coordinators and head coaches (I’m sure Bill Belicheck already has some ideas) in the NFL. If Kelly’s offense works, it will because he knew how to adjust to NFL offenses, not because he has a magical system that is unstoppable. The most likely scenario is perhaps an unexciting one: that the offense will do well, not crash, but certainly never reach the heights of the Ducks' offense last year.

Jay Cullen is a New York Giants writer for www.RantSports.com. Like him on Facebook at Jay Cullen-Rant Sports or add him to your network on Google.