It's true that his form dipped a couple of weeks after arriving, but it's hard to prove that it was because of being overplayed. It might as well have been a mental issue, or just by no specific cause at all, an inexplicable momentary dip.
The other way around, you should also acknowledge the fact that KDB had been on a fantastic run in recent weeks, arguably the best run he's had at the club so far, got rested one match v Stoke, and all of sudden turned invisible/ineffective v Madrid.
My point since the beginning: rotation will sooner risk your team to lose momentum rather than do them good because of the extra rests ...