Given the Portuguese's desire to be reunited with his former player, it seems clear that he rates Matic highly. He was afterall, a mainstay in the midfield that won Chelsea two titles in three years, so why sell?
From Chelsea's point of view, it made financial sense and perhaps the writing was on the wall as early as last summer when Chelsea brought N'Golo Kante to the club, the former Leicester man's arrival, followed by that of Tiemoue Bakayoko this summer has relegated Matic from frist choice to third in just 12 months and in the case of Bakayoko, Antonio Conte feels that he has got a younger, more dynamic upgrade on Matic.
Mourinho has been eager to improve his options in midfield all summer and his search has been focussed on 'destroyers', players like Matic who do the simple things well and give others a platform to play. This more than likely has something to do with Paul Pogba's role at the club, the feeling among United fans since his big money return has been that he is clearly a talented player, though many felt he simply did not fit into a two man midfield, perhaps the addition of Matic means that United are effectively looking to reshape their engine room in order to get the best out of the French international.
A midfield trio of Matic sitting deepest, with Ander Herrera and Pogba in front of them would represent a strong core, with many in the United fraternity feeling that Herrera was somewhat stifled by the necessity to sit deep last season. Matic will allowing him to play a less cautious game, just as he played back in his Bilbao days.
This tactical switch would represent a move away from the 4-2-3-1 that Mourinho seemed to prefer last season to a more fluid 4-3-3, which would perhaps explain why United have not been linked with many, if any, No 10s.
Matic would provide invaluable balance in the United midfield given that he is a rare left-footer. Whilst his imposing stature seems to be a common theme in Mourinho's side, with new fellow new signings Romelu Lukaku and Victor Lindelof also boasting impressive physical attributes, that's without mentioning Marouane Fellaini and Paul Pogba.
Was Matic be that was missing in the Manchester United midfield and the signging the club needed to push them towards a title challenge?
He may not be the most exciting addition on his own, but his impact could be huge.