Here's the rub.
Roberts was never really a good fit for Berry's offense, which is primarily designed for a power back with limited blocking as most plays are run out of a spread formation with 5 receivers.
Roberts was able to make it work for the last 2 years on his pure ability to make guys miss. This year, for whatever reason, that ability just doesn't seem to be there anymore.
In a different offense however, that might not matter, especially if Roberts is getting ample blocking support. But the Bombers just don't run those kind of formations.
So, Roberts lost "something" so that he was no longer effective in the Bomber offense, but could still be effective in another offense, is about how I see it going down.