Moncada doesn't have a motivation problem. He has a back problem. That's why he's missed so much time, that's why he's lost power, that's why he grimaces when swinging, that's why his defense hasn't suffered.
Unfortunately, a back problem is way worse than a motivation problem, it's why he'll likely never reach his potential, and it's also why he's stuck here. No one is taking a guy making $20+ million/year with a bad back.