Without Matic, there is no link between defence and attack
In Nemanja Matic's absence, Daley Blind replaced the experienced Serbian - but struggled to replicate his teammates' influence in midfield.
Matic is the player who links United's defence and attack so well, by collecting the ball from the centre-backs and actively looking for runs to feed the attack-minded players.
It appeared as though United players were just looking to pass towards Paul Pogba at every possible opportunity, but the creative Frenchman performed poorly and there was no alternative.
Mourinho will have picked up on this and will realise Matic's importance to link the team together - something that Michael Carrick can also do effectively, but is not reliable enough fitness-wise to remain available all the time.