No longer the lung-busting striker that broke onto the scene as a teenager, the 30-year-old Rooney accepted in recent months that a future in midfield beckons.
It was a role Rooney performed for England at Euro 2016, having ended the season there at United under Louis van Gaal, although successor Mourinho believes the Red Devils will get the best out him in attack.
Those comments upon the Portuguese's unveiling raised fresh questions about how best to utilise Rooney, but Mourinho has big plans and a lot of trust in his skipper.
"I am quite surprised with so many question marks around this guy," the United boss said at the team hotel in Shanghai.
"I know that at the national team level you have this capacity of trying to make things complicated for yourself, so what happened around him in the summer was not a surprise for me.
"But in my club we are not going to have that. He is the club captain, he is the manager's captain, he is the players' captain.
"I trust him a lot. I think he's going to be a very important player for me and no problem for him.
"He knows that he has to work hard, he knows that but I think that is nothing new in his career. Everything he got I see nobody offer him, he had to fight for it.
"With me it is not going to be different. But, yes, he is my captain."
Rooney's position in the starting line-up may be in doubt, but his Old Trafford future never looked in jeopardy.
The same cannot be said for some of his United teammates, with speculation growing around a number of players as Mourinho shapes his squad.
Juan Mata's position has come under particular scrutiny having been sold by Chelsea when Mourinho was Blues manager in January 2014, but the Portuguese was keen to set the record straight and confirmed the Spain international has an Old Trafford future.
"Well again it is a strange question because I don't understand why he doesn't have a future," Mourinho said.
"First of all, I think maybe all of you misunderstood what happened in our previous club because I never told Mata to leave. I never pressed the club to sell him.
"It was his decision and only his decision and I don't like players that want to leave.
"That's why I think every player has a price because if one day a player wants to leave it is because he does not want to be with us.
"If the player doesn't want to be with us, goodbye -- be happy where you want to go.
"Juan at our previous club was not so happy because he was not my first choice at that time. He wanted to play every game.
"He had a fantastic offer from a big club like Man United, he wanted to leave. My previous club had a great offer, we thought that was financially very much acceptable.
"We were not with this old-fashioned idea of 'I don't sell players to my direct competitors inside of my own country'. We sold him to Man United and done. Nothing else.
"In this moment I arrive, he's here, he works hard. He knows that I know he is a good player and he has space in the squad.
"But again until the 31st of August the market is open for every one of my players that is not happy.
"But, to be fair, I think he is. I think he is happy and I am not expecting him to ask to leave. I am expecting him to do what he is doing now, which is to fight for a place.
"In the end there will be space for everyone, we have 38 matches in the Premier League, we expect to play a lot of them in the Europa League and domestic cups so a squad of let's say 22 players there will be a space for everyone. So, yes, there is a space for Juan."