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This is just one small situation as part of the big picture, but it will illustrate my point just fine. The Buffalo Bills determine that they need to shore up their run defense, which makes sense because they were last in the NFL in that category in 2010.
The Bills take the first step in fixing their deficiencies along the defensive line by drafting Marcell Dareus.
Bills general manager Buddy Nix determines that the Bills need to get bigger and more physical at inside linebacker, so the Bills invest two more draft picks to help out the run defense, taking Kelvin Sheppard in the third round and Chris White in the sixth round, so the Bills appear to be set with these additions.
Because the draft was ahead of the free agency period in 2011, (normally they are reversed), the Bills really don't know what they will be able to land in free agency, so they took the safe route with the picks.
Free agency then finally starts when the new CBA is completed, and Bills free agent LB Paul Posluszny gets a huge offer from the Jacksonville Jaguars that the Bills know is way more than they were willing to pay, so they let Posluszny walk. Things are looking better for both Sheppard and White.
Then mere days later, the Bills wind up signing ex-Packers LB Nick Barnett at a bargain rate (relative to the Posluszny deal) and then surprise themselves weeks later that they can sign another veteran in Kirk Morrison for a very reasonable deal as well. Now all of a sudden, things aren't looking so great for the two rookie linebackers.
Both Sheppard and White miss key portions of training camp due to injuries, and as a result their learning curve will be a challenge for the rest of the year because they will be playing catch up all year long.
Then there is another twist. The only undrafted rookie free agent that makes the Bills 53-man roster, is linebacker Robert Eddins. Eddins is 6'3" and weighs 248. He played ball at Ball State. Eddins never missed any time, and impressed the Bills coaches all summer long, so he makes the team.
To the extent that they receive a significant number of snaps in 2011 is very much unknown. Taking it a step further, if they don't make much impact in 2011, the Bills may opt just to keep the veterans and let the rookies take their act to another team in 2012 or 2013, which all stems from their limited rookie season.
Then there is the possibility that Eddins could overtake one or both of them and stays on the roster long-term. You can probably find similar situations on every NFL team, but that is life in the NFL.