City had dominated their local rivals in recent years, including a victory in the 2023 final as part of their historic Treble-winning season.
However, United defied expectations at Wembley in 2024, pulling off a shock win despite finishing 31 points behind City in the Premier League.
McCarthy stated to Ladbrokes: "When we went to walk and inspect the pitch (FA Cup final), at that moment, the disrespect from the City players kind of played a huge part.
"You know, when you stand there and you see the players, all little groups of friends here and there, the players are out on the pitch and soaking up the atmosphere. When we walked back towards the dressing room, a lot of the City players were discussing their trips to Ibiza after the game. So in their heads, they've already won the FA Cup and they're already thinking about going to Ibiza and that.
"I think that kind of infuriated our boys because, mate, we'd got a final to win. We have a final to play, and you lot are talking about your after party! Almost like in terms of they've already won it, and they are preparing the celebrations for it like they're playing against no opposition. When we got into the dressing room, the players were like, 'These lot are disrespecting us'.