Mourinho's stinging comment came in response to a question that did not even reference De Boer, only whether Rashford would retain his starting berth after a match-winning brace in Saturday's 2-1 comeback win against Liverpool.
He's a player, he's very young, he needs games, but he's so talented.
De Boer was in charge of five competitive matches as Palace boss earlier this season, losing his four in the Premier League but beating Ipswich in the EFL Cup before his dismissal on September 11. If he was coached by Frank, he would learn to lose because he lost every game.
The 20-year-old, who has been relegated to the bench for much of the second half of the season, marked his first league start since Boxing Day with two goals against arch-rivals Liverpool on Saturday.
'I try to give the best to the kid.
"But if you go to his numbers and how many games he played with me".
Frank de Boer has laid into Jose Mourinho
"We are not one of the best teams in the competition but when a team arrives in the last eight, anything can happen", added Mourinho.
"That way of playing, that is a different view than we are used to in the Netherlands".
Rashford is set to feature in England's squad for the 2018 World Cup in Russian Federation and might be granted a more prominent role than expected as Gareth Southgate waits to find out the severity of the ankle injury Harry Kane suffered during Tottenham's 4-1 win over Bournemouth on Sunday.
The 36-year-old expanded on the heart condition he had in September.
Asked if it would be a failure if United were knocked out, Mourinho said: "A failure?"
So, is it justified to say that Mourinho does not provide youth a chance when the stakes are so high that even one of the greatest managers of the modern era, Guardiola, can not offer a chance in his side to several talented City youngsters, while the Portuguese has given ample opportunities to three youth academy products?
Lingard, meanwhile, has shown that he can be an integral part of the United first-team and has bagged goals in crucial games.
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