Chelsea have failed with two bids for Rooney this summer amid reports United have no intention of selling the England international to another Premier League club.
Mourinho does not want to see the 27-year-old sold abroad and believes Rooney will be the 'final piece in the jigsaw' if he lures the forward to Stamford Bridge.
"To me it is not a difficult decision because you want very much to keep Rooney or you don't want to keep him?" said Mourinho.
"But that old-fashioned mentality of, 'I don't sell players to clubs in the same country' - I think doesn't help.
"Sometimes you push players abroad when you should be keeping them in your league because then you are contributing to making your league the best league.
"You see in Italy it happens every season without any problems. Filippo Inzaghi played for Juventus, Milan. From the big players, only Francesco Totti stayed for life. Andrea Pirlo - Inter, Milan, Juventus."
Mourinho, who is optimistic of completing a deal for Tottenham target Willian, added: "We are going to get a maximum of two players in this transfer window. If we get the second piece (Rooney), it will be the final piece."
Asked why Chelsea will bid for a third time, Mourinho said: "We make the bid and they say no again. What do we lose? One email."
It has been suggested that Mata could be a makeweight in the deal for Rooney and Mourinho on Friday insisted the Spain playmaker was not for sale
"That's hypothetical, because they didn't ask," Mourinho responded when asked about what would happen if United wanted Mata.
"The two answers we got from Man United is 'we don't accept the bid', 'we're not interested in selling the player'.
"That's the only official contact we had with Man United. Till Monday, we have nothing more to add to it."
Mourinho also cited his own example of selling William Gallas to Arsenal during his first spell as Chelsea manager as a reason why United should not be afraid of letting Rooney move to west London.
He continued: "Players are sometimes unhappy because they want new contracts and at other times they are unhappy because they want to leave and that is a different situation.
"Other times it's because they really want to go. I was so sad about William Gallas. He was fundamental for us when we won titles.
"He said he wants, he wants, he wants and we couldn't stop him, so we decided not to stop him.
"As you know, in this club it wasn't an economical problem, it was just the player wanted to go. Ricardo Carvalho and John Terry were playing central defence all the time and he wanted to be first choice.
"He was playing many matches in a position that wasn't his best position, he was playing left-back and sometimes Wayne Bridge was playing, so he wasn't happy. So we thought it was a fair situation to sell him in the end.
"I had no trouble selling him to a rival. It happens all the time in Europe."