When you ask if DJ will rebound or not it is kind of a leading question. He was actually a solid performer last year in what everyone believes was a bad year. He was a RB1 in most formats. So even in a "bad" year he was quite good. Just not otherworldly.
This offer boils down to what you think of Cohen. If you don't think there will be much difference between Cohen and DJ in PPR then it's probably worth it. I would keep DJ.
A lot would depend on what I think of my team and who you think will be there when you pick at #4. IMO, DJ's window for top flight production is closing (if it hasn't already). I believe at best, he has one more good year in him. I'd be tempted to move him now while I could. But again, depends on your view of DJ. I'm not as high on him as most.
I tried to make this decision a few years ago with McCoy. I ended up holding on to him, and yes, he gave me another year or two of solid production. But he ended up not even starting much of the time since I had built well around him. If I had it to do over again, I would have tried harder to move him.