However, Rooney, who scored in United's 3-1 victory against Bournemouth in their opening league game, urged his team not to take their foot of the pedal against City, who also remain unbeaten this season.
"It's early but you want to try to build momentum and especially going into the international break," Rooney told British media.
"To go there with three wins is important and with the next game being Man City it was a big win for us, even though it's so early in the season.
"We know whatever happens against City isn't going to define the season but it's always nice to go into that break with the three wins."
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Mourinho and City manager Pep Guardiola will renew their rivalry when the two Manchester clubs meet on Sept. 10 at Old Trafford, and Rooney said there was an air of excitement ahead of the eagerly anticipated clash.
"It's a game players, staff, media, fans, everyone will be looking forward to," he added. "It's a Manchester derby but also with Jose and Pep Guardiola at Man City.
"It's exciting for everyone and exciting to play in."
(Reporting by Nivedita Shankar in Bengaluru; editing by Sudipto Ganguly)
