Although the Michael Essien transfer isn't done quite yet -- there's a medical to be taken (which is non-trivial in this particular case, because Michael Essien) and personal terms to be agreed -- the general structure of the deal has now been confirmed by Chelsea:
Agreement reached with AC Milan for the transfer of Michael Essien, subject to a medical examination and him agreeing personal terms. #CFC— Chelsea FC (@chelseafc) January 24, 2014
So, it's definitely a permanent transfer, which means Jake's previous points about the Bison deal are correct. Financially speaking, this is a money-saving move to offload a player who wasn't helping the first team -- and a fairly helpful one at that. Emotionally speaking, it's goodbye to one of the best players to ever have graced the shirt.
Love you, Mr. Bison. Good luck in Milan -- if anyone can bring you back to even a shadow of what you used to be, it's them.