Exclusive by John Sinnott |
McCarthy will face United again in Tuesday's second leg after scoring twice in Porto's 2-1 first-leg win.
He told BBC Sport: "I was shocked by our spirit compared to them.
"I kept it secret that as a child I was a United supporter. I was surprised and disappointed that they were not like the team I had grown up watching."
McCarthy added: "We managed to completely outplay them, which was a big surprise to me."
Porto have lost just once in the Champions League this season - 3-1 at home to Real Madrid last October - and are unbeaten away from home in the competition.
McCarthy, 26, believes that solitary defeat was important for Porto's season in that the team's players have learned not to be intimidated by European opposition.
"Real Madrid have the upper hand over Manchester United. They're very consistent and they're playing well almost every game and not dropping points.
"They also have the edge over United in that they have more big-name players, which intimidates other teams.
"When you play against Real you are sometimes scared to even tackle. It automatically gives them the upper hand and teams tend to respect them too much.
"That was a lesson we learnt. We were not going to respect United on the field, only after the game. That was the difference as to why we beat United and not Real."
After United's defeat, manager Sir Alex Ferguson accused Porto goalkeeper Vitor Baia of "making a meal" of a Roy Keane challenge - the Irishman trod on the goalkeeper's back - which led to the United captain's dismissal.
On television Ferguson also complained about Porto's "diving".
But McCarthy defended German referee's Herbert Fandel handling of the game.
"Obviously I am not in favour of players getting other players booked for nothing.
"But that is why the referees are there. He handled the game fairly well and sometimes when players exaggerated he didn't give anything.
"On the television I saw Roy Keane did step on the goalkeeper. It was a little bit silly with only a few minutes to go.
"The referee was not far away and the linesman was also there to see what had happened.
 McCarthy says Mourinho has made him a better player |
"If it had been me doing that to Tim Howard, everybody would have said it was a deserved red card. Why should the referee go easy on a Manchester United player?
"I think they are going to miss Roy Keane a hell of a lot. I know what he means to United. That's a big gap to be filled and whoever is going to replace him will have a lot of responsibility.
"There are the not same without him in the side."
Ferguson's furious reaction to United's defeat in Portugal prompted Porto coach Jose Mourinho to taunt the United manager, comparing the size of the two clubs' budgets.
While United top the world rich list of football clubs, Porto do not even make the top 20.
But McCarthy, who is top scorer in the Portuguese league with 16 goals, has nothing but the highest praise for the Porto coach.
"He has improved me completely," said the South African international. "For any young player he is a brilliant coach.
"He tries to know the players' personality so if you have an off day, he knows how to deal with you."
McCarthy, who in the past has been close to joining Aston Villa and Middlesbrough, added: "We know it's probably going to be probably the hardest game in Porto's history to go to Old Trafford and knock out one of the world's best teams.
"We are just trying to be optimistic. If we go through it will be brilliant, if not at least we will have put up a good fight."
MMIX
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It's probably going to be the hardest game in Porto's history 
