Chelsea's two-time player of the year, Spanish playmaker Juan Mata, is set to leave the club this week for a fee of around £37-40m to rivals Manchester United. His replacement has already been signed in the shape of Basel winger Mohamed Salah for a reported £11m. Good business, right? Chelsea make a profit of at least £26m on a player José Mourinho had no interest in using and would probably have languished on the bench for the remainder of the season. He'd been out of form this season anyway, and never took his defensive responsibilities seriously. Plus, remember that missed penalty in the Champions' League final in 2012? Could have cost us the win. Clearly overrated. You'll be hearing a lot of that over the next few days as Mourinho apologists attempt to justify what could turn out to be a catastrophically bad deal for Chelsea. In the short term, the deal has one or two mitigating factors which might explain Chelsea's way of thinking, though still appears unwise on balance. Looking further forward, Mata would have to inexplicably turn out to be a flop of Fernando Torres magnitude for this to be anything but a phenomenal deal for United at the moment when they need it most. Read on for five reasons why Chelsea are making a huge mistake...