The Bears ****ed around with McClellin a remarkable amount - different staffs not being sure if he was a DE, OLB, MLB, edge rusher type, whatever, and he was bulking up and slimming down accordingly. Just a mess. That being said, having watched plenty of Bears football, there was really never any indication that he was even a starter-level talent much less top of the first round, he generally looked lost, and was 10x more likely to be completely washed out of a play than he was to be in position to make a tackle. He had an inexplicable 2013 game against GB where he got 3 sacks and broke Aaron Rodgers, but that is his chief professional accomplishment. Most of the Bear fan hatred toward him I'm sure existed because of where he was drafted...if he'd been even a 2nd rounder there'd be considerably less beef.
If anyone can wring anything out of the kid I would think it would be here with BB/MP as he does fit the mold of what they like at LB more than any of the schemes the Bears were running. That being said, sometimes a guy doesn't fit at a certain position because he's versatile and talented and you need to find the role or scheme for him...and sometimes a guy doesn't fit at all the positions you tried him at because he sucks.