John Stones headed home early in the second half to edge City in front before Fernandinho sealed the win with seven minutes of normal time remaining to set up a final clash with Tottenham Hotspur.
Leboeuf, the former Chelsea and France centre-half, was convinced City deserved the win but was disappointed with the roles of Harry Maguire and Aaron Wan-Bissaka in the two goals.
He felt Maguire could have reacted quicker to stop Stones heading in unmarked, while Wan-Bissaka should have headed the ball wide rather than down the middle of the pitch before Fernandinho guided the ball back into Dean Henderson’s goal.
‘Football is made of mistakes and that’s what it is. You concede goals because you make mistakes,’ Leboeuf told ESPN.
‘The thing is with Manchester City this season they don’t make mistakes.
‘Stones and Ruben Diaz have been absolutely fantastic. It’s not the case for Manchester United where on the first goal, I want to know what Maguire has to say about that.
‘He doesn’t react, the ball comes close to him and he doesn’t react.
‘The second goal, the first thing you know when you’re a defender is when you kick the ball away you never kick it in the middle of the park.
‘Manchester City deserved to win. Manchester United suffered physically because they ran after the ball most of the time. That’s what it is. The best team won.
‘It could have been otherwise if in the first half Manchester United scored it could have been a different story.’