Rooney will be teaming up with a manager that wanted him at Chelsea.
Rooney looks set to play up front under Mourinho after a spell in the centre of midfield under predecessor Louis van Gaal and he is excited at getting to work under his new boss.
He told Inside United, as cited by the official Manchester United website: "I've always respected him both as a man and a manager, he's a nice guy and one of the most successful managers of all time.
"I like to think I'm always motivated as a player but I'd be lying if I said I wasn't a bit more motivated when a new manager comes in. I'm sure all the lads will be happy to be working under Jose and also eager to impress.
"I've played against many Mourinho sides over the years and I can't remember any easy games, so I think that says it all. His teams are always hard to beat, tactically disciplined and, most importantly, successful.
"He'll bring all these qualities with him and I'm sure he'll have his own ideas, but one of the biggest assets will be his experience. He's managed a lot of successful teams throughout his career and all that experience will be vital."
Mourinho is certainly a man with incredible pedigree having already won three Premier League titles with Chelsea, as well as winning the Champions League at both FC Porto and Inter Milan during his career.
The role of Rooney will be interesting, especially given the strong links that Mourinho had with wanting to sign him at Chelsea during his second spell at Stamford Bridge, with The Guardian reporting that three bids had been tabled for the England international.