You want to get all your players, especially your top guys, about 55-70 minutes a week. Over 80 is a real danger zone and he'll almost certainly start dropping. Start him 3 times a week so that he plays over 100 minutes and this huge drop will happen every time. This is why depth is important, the game engine doesn't let you ride a couple of star players like this.
You want to keep gameshape at a minimum of respectable but you'll notice that all the strong teams in BB keep all their players at strong-proficient. That's how you maximize their abilities.
You need to plan ahead to balance your minutes. Don't play him in scrimmages at all as they don't mean anything. If you're playing a team that you could blow out, a team you can't possibly beat or a computer run team, that's a good time to sit him too.