The Manchester United manager made the curious comment during a news conference ahead of Saturday's lunchtime kickoff when he also played down the significance of a match in which he will lock horns with City boss Pep Guardiola for the 17th time.
Both managers have enjoyed 100 percent starts in their new posts, winning all three games in the Premier League.
But while Mourinho has a full cast list at his disposal, Guardiola will be deprived of Aguero's lethal firepower after he was banned for three games following an elbowing incident against West Ham United.
Aguero, top scorer for City every year since joining in 2011, has scored six goals this season and has seven career goals against United, but Mourinho refused to accept his unavailability tipped the balance his side's way.
"Yes it changes because it is more difficult," Mourinho said. "When Aguero is available we know that he plays, we know their formation and their dynamics - nobody has doubts.
"If he is ready, he will play. But now he doesn't play, they have an amazing squad in terms of options and there are many things (Guardiola) could do."
Leroy Sane, the 20-year-old winger signed for 37 million pounds ($49 million) from Schalke, is one option for Guardiola who has suffered only three defeats to Mourinho teams.
The Portuguese knows Guardiola will have something up his sleeve, but insists that players, not managers, will decide the outcome of the game.
"He could decide to play with (Kelechi) Iheanacho or (Raheem) Sterling," Mourinho said.
"He could decide to play with Silva as a fake number nine between the lines. It is clear that they have many options," Mourinho added.
"That for us, in the preparation of the game, makes us have to go step-by-step with every situation because we are trying to reduce the unpredictability of the game.
"To try to do this now is more difficult."
Mourinho said that so early in a long season no match, even one against City, was more important than any other.
But landing an early jab on the chin of Guardiola would be an irresistible carrot for the Portuguese who has transformed the mood at Old Trafford since his arrival, following the departure of the unpopular Louis van Gaal.
With Zlatan Ibrahimovic amongst the goals and Paul Pogba adding much-needed zest to United's engine room, they look slight favourites to take the points.
"We are ready, we are waiting," Mourinho said, refusing to get drawn on the inevitable spotlight at the renewal of hostilities between himself and former Barcelona boss Guardiola.
"I know what the derby means to the fans but I have to stay under emotional control," he added.
($1 = 0.7539 pounds)
(Editing by Ed Osmond)
