Alex Ferguson has claimed he will retire if he can win "a couple more European trophies" with Manchester United.
In an interview with CNN, Ferguson conceded that United should have enjoyed more success in Europe than the two Champions League titles in 1999 and last season.
"If there was one challenge I'd love the club to win more European trophies than they have done and I think I've made this point a few times over the years. I don't think we've won the number we should have won," he said.
"There have been years when we've been unlucky and years we've been disappointed, and there've been years when we've thrown games away, and that sort of gives a kaleidoscope of what football's like.
"If we could win a couple more European trophies I then would definitely quit."
Ferguson also spoke about star winger Cristiano Ronaldo, praising the 24-year-old for the maturity he has developed in avoiding the temptations that come with his high profile.
"I think that he's had temptations and I think there have been periods when you've seen the effect of it (on the pitch)," Ferguson said.
"He's lucky in the sense that people here can tell him the truth and be straight with him and he appreciates that, and he has responded to that.
"If it stays with him and changes him as a person then you have a problem.
"Fortunately Ronaldo has good human qualities. He's a good guy.
"He knows when he's gone too far and he draws back in after that."
United return their attention to the Premier League with a trip to Sunderland tomorrow, and to try to bury the memories of their disappointing 2-2 Champions League draw with Porto.
Scottish midfielder Darren Fletcher said: "The mood is that we are still in every competition and we are still battling to win them.
"Whether we can do it or not is going to be a huge task.
"It was massive for us to win on Sunday because Liverpool have come back on form and they had scored in the last minute against Fulham.
"The character we showed against Aston Villa to come from behind to win the match was incredible.
"We are disappointed about the Porto match but we have to pick ourselves up as we have a tough game against Sunderland, who are fighting for their lives against relegation and have everything to play for."