Sir Alex Ferguson admitted United benefited from a large slice of luck to launch a match-winning spell of five goals in 20 minutes.
Goals from Darren Bent and Luka Modric put us 2-0 up at Old Trafford and we looked comfortable 10 minutes into the second half, despite the arrival of Carlos Tevez at half-time.
But it all changed in the 56th minute when referee Howard Webb pointed to the spot when Heurelho Gomes had clearly palmed the ball away from Michael Carrick.
Ronaldo converted the penalty and sparked United's revival -10 minutes later and they were 3-2 up before winning 5-2.
"We were a bit fortunate but football is a funny game," said Ferguson.
"You get terrible decisions. We got one last week (in the FA Cup semi-final). You have to deal with them and get on with it.
"In the first half we were playing far too slowly," he added. "But bringing on Carlos Tevez enthused everyone, the fans included. He won the game for us. He turned the game, although obviously everyone played their part.
"Wayne Rooney, as well, has such desire and it doesn't matter where you ask him to play. He has wonderful hunger and drive that can transform a team and a game."