There's no doubt that DeSean Jackson is coming off a down year, but it seems to me that a lot of Eagles fan have gone too far in discounting what he actually does for the team. The numbers he's put up and the way he's tallied them over the first four years of his career are actually historic.
Take a look at the rare company his production has put him in. He's just the 5th player in NFL history to record at least 900 yards receiving in his four seasons. The other guys? Randy Moss, Jerry Rice, Joey Galloway & Bob Hayes.
ESPN's Stats & Info blog has some more on the kind of unique playmaker Jackson has been.
Since entering the league, Jackson has an NFL-best 25 receptions of 40 or more yards and a league-high 14 touchdowns of at least 50 yards. He’s scored long touchdowns at an historic rate; Jackson’s nine 50-yard touchdowns in 2009 tied the NFL single-season record held by Devin Hester (2007) and Elroy Hirsch (1951).
Jackson also has the ability to score from all over the field. He’s scored at least once on a rush, reception and punt return in three of his four seasons. The only player in league history to with more seasons with a rushing, receiving and punt return touchdown is Eric Metcalf, who did it every year from 1992 to 1995.
Now, this isn't to say he's worth Larry Fitzgerald money or anything... Obviously what he gets paid is still the big issue, but we can't act like losing him wouldn't have been a big deal. Even in his worst year, he still contributes a lot of production that would be tough to replace.