Different players, different circumstances. Like comparing apples and oranges. He had one absolutely fantastic year and was at a great age. There was also no indication to me that he lost that talent as he still performed during the time when it was rumored Mourinho didn't trust him. We had every reason to believe their manager was an idiot for selling. Turns out he might not have been that stupid since Mata has never recaptured that one season where he was unplayable.
Now we have Matic, 29, a man who can "do a job in Mourinho's system" (have we had enough of them yet? Men who can do a job for us?), who has had one good season that was three years ago, a player who i feel was decidedly average last season (unlike mata that season at chelsea), this guy definitely classes as a reject yes.