United’s semi-final woes continued at Old Trafford on Wednesday night, having been dumped out of all three cup competitions at the same stage last season.
City inflicted a 2-0 defeat on their Manchester rivals thanks to second-half goals from John Stones and Fernandinho, meaning Ole Gunnar Solskjaer’s wait to contest a final goes on.
And Leboeuf believes the Red Devils boss was let down by defensive errors from Maguire and Wan-Bissaka on the night.
“Football is made of mistakes and that’s what it is. You concede goals because you make mistakes,” the former Chelsea centre-back 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. You kick it on the side. Wan-Bissaka did it straight to Fernandinho’s feet.
“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.”
Despite suffering a fourth straight semi-final exit, Solskjaer denied that his United players have a psychological problem with getting past the last four.
He told Sky Sports: “It's not psychological. Sometimes you meet good teams in semis and we just didn't have enough. We met probably the best team in England at the minute and for me we just didn't have enough today.
"We wanted to go to a final and we wanted to look forward to it because it would have given us an extra boost.
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“But it's not going to happen and we've got to dust ourselves off and use tonight to get the disappointment away, because on Saturday we've got the FA Cup, then some big league games against Burnley and Liverpool.
“Then within a month the Europa League is back. There's lots to play for.”