And Mourinho believes Matic's consistency will prove to be vital for United as they aim to launch a Premier League title challenge.
"Matic was a big player for Chelsea last season and even though they have sign players to replace him, he is going to be missed and possibly just as significantly, he strengthens Manchester United, there is no doubt about that".
"He's a player of, I think, now 29 last week - 29 years old, with five years or so in the Premier League, lots of experience".
'We have to rest well because we have a very important game on Sunday against West Ham, ' Matic said.
"In our period of dominance he was what we want him to be - to move the ball in a simple way, to give balance to the team, to start building from the back".
The Serbia worldwide played a key role in the Chelsea team that won the top-flight title with two games to spare in Antonio Conte's first season last term.
And when asked for his thoughts about talk of a move away from United, he said: "I'm pleased that Manchester United consider me to be off the market, just as I'm pleased by the interest of Italian clubs".
'Victor can play right back and can play central midfield, we got a central midfielder [Matic], we got a striker [Lukaku], obviously the 25% would be a player for another position playing from the sides.
'We had an opportunity to equalise, Marcus (Rashford) had a good chance and didn't score, unluckily for us, but this is football'.