OK. I'll repost these stats.
Let's use FootballOutsiders metrics, which account for targets and usage. Let's use Welker for frame of reference. Keep in mind 2010 was Welker's worst year coming off the ACL.
Julian Edelman 2010 DVOA: -29.1%
Julian Edelman 2011 DVOA: -46.8%
Welker 2010 DVOA: 4.1%
Welker 2011 DVOA: 20.4%
AdvancedNFLStats metrics, which also accounts for targets and game situation:
Julian Edelman 2010 EPA (Expected Points Added): -.04
Julian Edelman 2011 EPA (Expected Points Added): -17.5
Welker 2010 EPA (Expected Points Added): 36.8
Welker 2011 EPA (Expected Points Added): 115.8 (led all receivers)
Julian Edelman 2010 Success Rate: 44.4%
Julian Edelman 2011 Success Rate: 33.3%
Welker 2010 Success Rate: 57.1%
Welker 2011 Success Rate: 66.2%
Short of going back and watching every snap these guys took, this is as close as you can get to what you want to see. And this is what it shows.
If you took Edelman's EPA per play in 2011, for instance, it is -.83, which is so bad it doesn't even register on their website, since the worst receiver they show is Andre Caldwell at -.39, which itself is historically low compared to other seasons.
Again, if we go back a year earlier, we see Edelman was a pretty effective slot receiver. 2009 stats: -.5% DVOA; 15.5EPA in limited play, which is good; 52.3% success rate. But he hasn't done that since.
Conclusion, and I don't know how to make this any simpler:
Edelman played well in the slot in 2009. He has performed rather poorly in limited snaps since.
What is there to argue with there? We all hope the guy improves and becomes the jack-of-all-trades we want him to be. But to deny his noteworthy ineffectiveness in the last two seasons is foolish.