"It will be a change for him, but I wasn't the same player at 24 as I was at 28".
And Mourinho believes Matic's consistency will prove to be vital for United as they aim to launch a Premier League title challenge.
"I heard whispers at the back end of last season that he wanted to go and even if the club can not come out and say too much about that, maybe there was an element of that in play here when this deal went through".
The midfielder joined United from Premier League champions Chelsea in a deal believed to be worth £40m in July.
The 29-year-old was also a key part of Mourinho's Blues when winning the league in 2014-15 and has seen enough in his short time at Old Trafford to believe this will be a successful season.
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West Ham have also been active in the transfer market after flirting with the relegation zone last season and could feature Mexico worldwide and ex-United striker Javier Hernandez in attack on Sunday.
Matic was one of Manchester United's best players in an otherwise disappointing 2-1 loss to La Liga giants Real Madrid in the Uefa Super Cup.
"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".
'There is a lot of games, it's a long season but I expect us to do well'.
'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 had a good chance and didn't score, unluckily for us, but this is football".