They basically flipped a guy who didn't want to be there for a position of significant need. They are in dire need of more skill positions on offense, and at 6'4 238, they don't have anyone else like Claypool on the roster. He's a compliment to Mooney.
That said, if Fields hadn't started to show signs of potential the last two weeks, I doubt they're making this move.
This season is still pretty well lost, but if they spend the off-season focusing on acquiring oline help, they could take a step forward next year.