The Red Devils striker was punished due to a lacklustre showing in training following a night out after the club's Boxing Day victory over Wigan.
Rooney missed United's next match against Blackburn on New Year's Eve and produced a subdued display on his return in last week's 3-0 defeat at Newcastle.
However, the England international returned to his best with a brace in the FA Cup win at local rivals Manchester City after acknowledging he was 'tense' after being punished by manager Sir Alex Ferguson.
When asked if he had grumbled about the fine, Rooney told La Repubblica: "Yes, but for reasons that I can't say. I accepted it. But I was tense when I got back on the pitch and I think it showed."
Rooney added: "You can't always agree. Football is made up of all kinds of conflict.
"In a dressing room, between players, between us and the manager, between us and loads of people who don't seem to matter. It's constant and harsh sometimes."