I'm not privy to Chelsea's transfer plans. Obviously.
You can however read their financial reports whenever you like and if they want to make ffp, which it appears they do, then they have to sell. Getting £40m for an unwanted bench warmer is great business and gives them alot of freedom either now or in the summer.
You seem to be hung up on your opinion of Mata's quality, however your opinion means nothing to Chelsea who have demonstrated time and again that Mata is surplus to their requirements.