Some players improve when they're around better players and that is what most hoped for when he came here. I was with the bunch that thought he deserved a break after playing like 70+ games in a season plus IIRC at a young age. The "burnout" excuse was valid a few years back, but is overall game hasn't improved much during his time here IMO. Some might have expected too much from him.
He just hasn't delivered on a consistent basis and it shows it's one of his character traits. The decision to get rid of Mata AND KDB has backfired big time, especially with the latter excelling so much in half the time he's been given.
Chelsea need to cut their losses and cash in for everyone's benefit.