#5 Weakens rivals

As much as it is important to focus on the team than the opposition, there is no doubt that buying a player wanted by a rival is a double blow. Firstly, it strengthens the team and secondly, the rival misses out and might find it difficult to find an alternative. In Matic's case, the player was directly bought from the defending champions Chelsea.
This represents an even bigger effect as Matic was key to Chelsea's title win in the previous season, where United finished sixth. Further, Chelsea boss Antonio Conte is yet to adequately replace him, with AS Monaco's Tiemoue Bakayoko the only midfield arrival.
Now, considering the fact that Chelsea did not compete in Champions League last year and their thin squad was relatively free from injuries, new signings were needed for this season. However, selling Matic not only deprives the Blues of squad depth but also has a negative effect on their European hopes.
In all of this, United and Mourinho get a player they want and need. Thus, Matic's signing is more than a double blow as it increases the chances for the Red Devil's to have a more successful season.
One can see from the above reason how Matic ticks every box one can expect from a transfer. While he may not be the most expensive signing this summer, there is no doubt he is the signing of the summer so far.