Every hitter/pitcher has their ups and down so if you cut bait on everyone when they had a slip then you wouldn't have a roster. To think YRod was incapable of turning it around when you paid to lock him up longterm is a confusing decision.
Look at our closer if you need an example, YRod should continue getting the reps even if its not all 100% leverage situations. MLB seasons are a marathon, the best always rise to the top by the end.