The FA confirmed on Thursday that Rooney will miss the Wembley semi-final clash on April 16 after it upheld a two-match suspension for his foul-mouthed outburst at West Ham last weekend.
But Mancini has denied City have cause to celebrate Rooney's absence, and said: "I don't think it improves our chances. We know Rooney can change a game in any moment like at Old Trafford where we deserved at least a draw. But the team is important."
He added: "When there are important games it is better all the good players play but at the start of the season Rooney did not play and United were a strong team."
Rooney is convinced he is being made a scapegoat by the FA.
Twelve hours after he revelled in his matchwinning effort in Manchester United's Champions League quarter-final with Chelsea at Stamford Bridge, Rooney discovered there had been no leniency shown by the FA for his foul-mouthed outburst at Upton Park on Saturday.
In issuing a public apology so soon after the final whistle and being the father of a young child himself, Rooney understands the general vitriol. What he cannot fathom is why previous instances of public swearing have gone unpunished whilst he must now miss two matches, one of which is going to be such a huge occasion.
"I am not the first player to have sworn on TV and I won't be the last," he argued.
"Unlike others who have been caught swearing on camera, I apologised immediately. And yet I am the only person banned for swearing. That doesn't seem right."
Source: PA
Source: PA