When we evaluate and compare players, we do our best. It is not an easy thing, though. It is highly subjective because so many guys in football provide value in so many different ways, even guys who play at the same position. Let me give an example.
Ever since the 2009 season, Darrelle Revis and Charles Woodson have been linked. They were the two top contenders for Defensive Player of the Year. The voters
committed highway robbery on Revis gave the award to Woodson. It seems natural to compare the two guys because they both play cornerback. They really only play the same position in name only, though. The way their teams use them are completely different.
The way the Jets use Revis is simple. He usually follows around the other team's best receiver all game and sticks to him one on one. Darrelle is a unique weapon. Against say the Texans, most teams either have to accept that Andre Johnson is going to do damage or dedicate two people to taking him away, which opens things up for other offensive players.
Woodson is used as more of a playmaker for the Packers. They move him around, shifting him to create favorable matchups. He lines up against receivers. He sometimes takes snaps at safety. They like to play him at linebacker, where his good run support allows the Packers to get away with having extra secondary players on the field to play the pass. Woodson is a good cover guy but is not in Revis' league. His value is derived from a number of different sources, good but not spectacular coverage, intercepting passes , forcing fumbles, blitzing, and run support. Once Tramon Williams and Sam Shields emerged in 2010, defensive coordinator Dom Capers used him a lot lined up against the slot receiver so he had a shorter path on blitzes and run plays.
Even at a position like corner, it is difficult to compare Revis and Woodson because they bring value in different ways.Some people say that since Woodson lines up in more spots, he was more valuable. That is not a clear cut argument, though. Say a guy does three different tasks at one company. He might bring in $9 of revenue. Another guy might do one task and bring in $10. One guy does more different things, but the other guy is more valuable. An MVP candidate in baseball might play 150 games at shortstop. Another might play 50 at shortstop, 50 at second base, and 50 at first base. The first guy is more valuable because shortstop is a more difficult position. Same concept here. Revis stuck on the best receiver on the other team. Woodson played a lot at safety and in the slot, which are easier because the competition is not as good.
Back in 2009, Woodson won the award in no small part because he was more of a big play guy. Anybody can see and remember a guy getting a sack and an interception. Appreciating what Revis does requires a more nuanced approach. You probably won't notice Revis' coverage on Johnson unless the ball is thrown at him, but single handedly taking him away does so much for a defense. It eliminates a dynamic threat. It allows the defense to give extra help to secondary receivers. It gives pass rushers extra time to get to the quarterback since the primary target is covered, and the passer must take extra time going through his progression. You cannot discount Woodson's big plays, though.
I'm not looking to relitigate what happened in 2009,. My point is more the difficulty this provides in ranking players even at nominally the same position. We see it all the time. This is one example. How about the difference between outside linebackers? A 3-4 one like DeMarcus Ware does things differently from a 4-3 one like Chad Greenway. Both are really good, though. They have different responsibilities.
Now factor in comparing guys who play different positions like the one on NFL Network right now. It gets even more subjective. That's why player rankings are fun but ultimately more about starting a discussion than proving to be definitive. In basketball, there are only five players on the floor, and everybody gets the ball a lot. In baseball, everybody at least hits. In football, everybody is working within the confines of a specific role, and there are a lot of different styles. By now, most people seem to agree Revis is better. When you really break it down, it's tricky in many cases like this.