Its utterly bizzare that he is retiring, I was expecting him to coach until he croaks in the dugout.
If this isnt the perfect timing for Mourinho to go there, I would be utterly shocked if he isnt announced as the new Man U coach in the coming 2 days. His dream job, and him and Fergie are pretty much buddies nowadays.
If for some bizzare reason not him, see Moyes or an outside chance for Klopp, but both unlikely.