Mata goes missing in games more often than he helps win them, he played right side at Valencia and at United, so basically 2/3rds of his career has been spent playing coming in off the right flank while people still make out hes a dyed in the wool no.10? Willian has had a much better individual season this year than Mata , his stock is high, Mata's is low.
Personally, I don't see why Chelsea would entertain a trade between the two, considering he (Willian) has been their best player all season, but if they'd be interested in taking Mata back i'd be all for it, especially with Mourinho coming in who sees Mata for what he is, a luxory player.