United face City's title rivals on Sunday.
Guardiola said: "Will I support United? I'm a City fan and the objective is to beat Fulham.
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"That's all I worry about."
On Mateo Kovacic's first season with City, he added: "He played very well because he doesn't have Gundogan's characteristics and I used him out of position. He was really good against Nottingham Forest, for example. To recall the last few games he was extremely important, he changed our game."