After Marcus Rashford bagged a brace in United's 2-1 win over Liverpool on Saturday, De Boer took aim at Mourinho.
"Frank respects Jose a lot and was simply pointing out that, if Rashford was playing for Ajax and had one or two bad games, he would still start the third game".
"When you are younger than I am, and you are learning the game, I think you do need it. But he's so talented and you want to see him play every week".
The United boss had an acerbic response to hand when he addressed a news conference ahead of Wednesday's (AEDT) UEFA Champions League showdown with Sevilla, seemingly holding De Boer responsible for Palace's slump extending to seven winless and scoreless matches as Roy Hodgson picked up the pieces at Selhurst Park.
"If he was coached by Frank, he would learn how to lose because he lost every game".
"Right now, I'm probably not learning the most on the pitch", said the two-goal hero.
South Africa apologise to Australia on Sonny Bill Williams mask incident
Despite the distractions, Warner has been Australia's top run-scorer in the series so far, with 142 runs at an average of 47.33. Meanwhile, an actual game of cricket is going on, with South Africa 7/263 in reply to Australia's 243 on stumps at day two.
"What he was saying was it was not good for Marcus Rashford to have a coach like me because the most important thing for me is to win".
In a man-of-the-match performance against United's bitter rivals, Rashford scored with both of his shots on goal. Of course, with Marcus, I have to be honest and give the credit to the people in the academy who were responsible for his formation; I have to leave credit to Louis van Gaal, who was responsible for his first season in the first team.
Mourinho believes United will have a chance of winning the trophy if they progress.
"When you are in the last-16 it looks really, really far but when you are in the last eight you start smelling the semi-finals, the feeling is that you are amongst the best".
"We are not one of the best teams in the competition but when a team arrives in the last eight, anything can happen", said Mourinho.
Paul Pogba returns to the squad after a minor injury, but the club's record signing must do with a place among the replacements.
Michael Carrick has confirmed he will retire at the end of the season. "That's pretty much where I'm at".