City captain Kompany was given his marching orders after just nine minutes on Sunday, after appearing to make a two-footed tackle on United winger Nani. The dismissal proved crucial as United went on to claim a 3-2 victory against their opponents - but Mancini believes Rooney played a key role in Foy's crucial decision by making his feelings known.
"It was not a red card - Rooney told him his decision," Mancini said afterwards.
When pushed further, he added: "Yes, he [Rooney] was near the referee. It was not a red card. It's impossible, but I don't want to talk about the referee because I want to stay on the bench for the next game."
Rooney later dismissed the accusation, however, posting on Twitter: "Funny how people think I got Kompany sent off. Im not ref. i didnt give red card. But it was a clear red card. 2 footed tackle [sic]."
The Italian confirmed the club will appeal against Kompany's red, which will otherwise see the Belgian miss four games having already been sent off once this season. Mancini is confident of a successful appeal, despite similar cases - most recently Nenad Milijas' red card against Arsenal - seeing the referee's initial decision upheld by the FA.
"We will do an appeal and I am sure that we will win," he said. "There is no way Vincent shouldn't play against Liverpool in the Carling Cup semi-final on Wednesday.
"It was 300% not a sending off."
Despite losing the game at Etihad Stadium, Mancini revealed his belief that the manner of his side's performance after they went down to ten men will stand them in good stead over the remainder of the Premier League season, believing the league leaders have gained a psychological edge over the side three points behind them.
''This evening is important for the league," the former Inter Milan boss noted. ''Now I think we can win the league because of the character we showed. United is a top squad in England.
"At Old Trafford, with ten men for 40 minutes, they lost 6-1 [in the league]. Today we played for 80 minutes with 10 players, scored two goals and didn't concede any chances. It was a fantastic performance.
"If we have the same attitude and same strength every game, we will win the league."
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