Matic joined Manchester United from Chelsea in a £40m deal last month after being frozen out by Antonio Conte.
He admits Chelsea are a “big club” but believes United are on another level.
When asked if United are the biggest club he’s played for, Matic said: ”Definitely, everyone knows what Manchester United is about.
“When you say Manchester United, it is different. Also Chelsea is a big club, Benfica are the biggest club in Portugal. But Manchester is Manchester.
“When I came here for the first day, I could feel that the people at the club help you every day if you need something and supporters are very important for this club.
“After every training session, they are waiting for us outside the training ground to take pictures, to give a signature so I think the club is one of the biggest in the world.
“Maybe you have two or three clubs similar to United, but Manchester is one of the biggest clubs in the world for sure.”
“He bought me to Chelsea from Benfica and now when I decided to come to Manchester, he was a reason to come here.
“Mourinho was the reason and so was the club, Manchester United, so you don’t need to think a lot about if you are going to come.
“My decision was very quick and very clear.”