It was announced a while back that it was happening.
He's going to Athletico on a free, but you cant loan a loan player, so Chelsea have to sign him over to AC, who can then loan him out (outside the transfer window) then he'll complete his free transfer when the window opens.
Its just a way for him getting there quicker - but there must be *SO* many caveats in his new contract...
Id laugh if Athletico end up not wanting him (if he turns up and is tosh) - and AC Milan are stuck with his wages (instead of Chelsea)
There must be so many get out of jail lines in his contract