The Brazil international feels the Blancos did enough to earn victory over two legs, and stressed that referees do not have an easy job following Nani's dismissal
The Santiago Bernabeu side were held to a 1-1 draw in the first leg of the tie, but turned things around in the return as they recorded a controversial 2-1 win at Old Trafford on Tuesday evening, but Kaka feels Madrid were the better side of the two over 180 minutes.
"The result of the tie is deserved and going through gives us confidence to get the 10th title. We knew the importance of the match, like we did against Barcelona.
"We have not won anything yet but it is a step forward. I am happy with the team because I know how hard it is to play here and to knockout Manchester United.
"I'm happy that we managed it. In football you never know what will happen and we still have a chance to win a double."
The attacking midfielder also had his say on Cuneyt Cakir's decision to dismiss Nani after a reckless tackle on Alvaro Arbeloa, and defended the referee's call.
"I don’t know if it was right or not, I'm not sure what happened. Referees have to decide very quickly, though, and it is hard for them. Playing with 10 is not easy.
"There was also a handball in the area from a shot by [Gonzalo] Higuain. These things happen in matches and you have to move on."
Madrid resume match action on Sunday when they travel to Celta.