Chelsea will return with a third bid for Rooney following Monday night's Barclays Premier League meeting between the sides, despite being told the striker is not for sale.
It was put to Mourinho that he will not be the most popular man at Old Trafford in a match where there is an opportunity to strike an early blow in the title race.
"Why? They are against me?" Mourinho said.
"But I didn't say (to Rooney) you will be a second choice for me. And they are against me?
"We are trying to get a player that the manager told 'you will be a second option'. We are not going for (Robin) van Persie.
"They don't have to be against me. If I say Ramires is a second option for me and he plays when (Frank) Lampard is tired or injured, if somebody comes here to get Ramires, nobody is upset."
Asked if it is Moyes who is culpable for the Rooney situation, Mourinho said: "Of course."
If Mourinho, who expects to complete a deal for Willian imminently after hijacking Tottenham's move for the Anzhi Makhachkala forward, can prise Rooney from United by the September 2 close of the transfer window, it would be a significant coup.
But it would also lead to the departure of one of Chelsea's existing three strikers - Fernando Torres, Romelu Lukaku and Demba Ba.
Ba appears most likely to leave, even if only on a temporary basis.
Mourinho said: "I told the players that I want a squad of 22 (players).
"They are professionals. They want to stay. (But) they know that, if somebody's coming, it will be hard because, normally, somebody has to leave."
The arrival of Willian would also result in a departure, with Chelsea having a host of options for the three attacking midfield positions in which he is earmarked to play.
Victor Moses' place at Chelsea is most precarious.
Mourinho insists it is not solely his decision on who leaves, whether it is a striker or another forward.
He said: "This is not about choosing one to leave. It's to agree a situation.
"If a player does not want to leave, the player does not want to leave.
"But we are speaking hypothetically because maybe we don't get another striker and maybe we keep the three strikers we have."
It was put to Mourinho that Chelsea's last-gasp pursuit of Willian was designed to unsettle Spurs and was a personal vendetta between the Blues boss and his former staff member Andre Villas-Boas, now Tottenham boss.
"No, no way," Mourinho said. "Purely business."