At least not at the level we're all assuming. Sure, he hasn't had a great year, but I would argue that he didn't have one last year either. He had a LOT of touches, and capitalized on those touches to the best of his ability.
Last year: 315 Car, 1231 Yards, 8 TD, 3.9 avg. 64 Rec, 486 Yards, 4 TD.
This year: 201 Car, 685 Yards, 4 TD, 3.4 avg. 51 Rec, 454 Yards, 0 TD.
Which puts him on pace for: 247 Car, 840 Yards, 6 TD, 3.4 avg. 63 Rec, 552 Yards, 0 TD.
Which would put his total output at-
08: 379 Touches, 1717 Total Yards, 12 TD. (4.5 yards per touch.) (1 touchdown per 31.58 touches.)
09: 310 Touches, 1392 Total Yards, 6 TD. (4.49 yards per touch.) (1 touchdown per 51.66 touches.)
# of 100+ yard games
08': 3 rushing (123 @ IND, 126 vs DET, 139 @ STL), 11 total.
09': 1 rushing (121 vs DET), 5 total (and one which was 3 yards off).
Aside from his touchdowns (which can be attributed to the both the increased passing threat and shoddy o-line), where exactly is he THAT much worse? I guess one could say that he has less DECENT games, which is true, but that can partially be explained by a general lack of touches, or at least at the massive amount he recieved last year.
It's not like he went from Chris Johnson to Chris Brown. He had a very productive year last year, but he rarely took over games. We have to accept what he is, which is a good recieving back with average speed and average power. He's not particularly bad at anything, but he's far from an elite running back (even last year).
Granted, he has fumbled much more than he should and seems much less effective from the good old eyeball test (actually WATCHING his play). But he is FAR from the worst ailment of this team. In my opinion, the biggest injury of the season (aside from Urlacher) was when Kevin Jones went down in the preseason finale. A healthy Jones (and a healthy Garrett Wolfe, who was showing an improved overall game this season) could help a lot.