The Dutchman, who is still without a job, served as a pundit for the United-Liverpool clash, and claimed it was a "pity" that Rashford has Mourinho as his manager after the game.
De Boer had praised Rashford after his impact in Saturday's win over Liverpool and said it is a "pity" the 20-year-old plays under Mourinho at Old Trafford - prompting an angry response from the United boss.
On Saturday, Rashford marked his return to the side with two goals against Liverpool, and Jose Mourinho has made a decision to reward the England worldwide with a second outing in three days.
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Not trusting young players is a stick Mourinho has been beaten with since his first spell at Chelsea but a stat has been unearthed that will change your mind on everything you've heard about Rashford recently.
"You can't be depressed around me, I'm always winding him up".
"If he [Rashford] was coached by Frank, he would learn how to lose because he lost every game", Mourinho said.
"We try to give the best to the kid's education".
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"Mourinho has bought (Alexis) Sanchez in January and of course it's the truth that it's not good for Rashford's development and he will play less now for Man United".
"He was saying it is not good for Marcus Rashford to have to be coached by me".
After the academy product's performance vs Liverpool it will surely be easier for the Portuguese manager to include his player in more matches.
Mourinho believes United will have a chance of winning the trophy if they progress. But when a team arrives in the last eight, anything can happen. Mourinho said he did not know whether the midfielder would be available but Pogba had trained after the open part of Monday's session closed.
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LaBeouf was arrested in Georgia after his request to bum a cigarette escalated into a full-blown racist rant. So I went after the dude with a knife [but didn't use it].I've always thought somebody was coming in.
"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". "That's pretty much where I'm at". "It's kind of been sorted out but we're still talking about it so there is nothing to totally confirm yet", he said.