And the United boss took time out of press conference on Monday to lambast the Dutchman, calling him the "worst manager in Premier League history".
De Boer is not the only pundit to question Mourinho's conservative approach when it comes to Rashford who is vying for a starting place alongside the likes of Romelu Lukaku, Anthony Martial and Alexis Sanchez.
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'If he has one or two not good games, he puts him out.
"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".
"Tomorrow is a very hard match, playing against a team that didn't lose against Liverpool home or away [in the group stage], that drew in Liverpool and I say Liverpool because they are an example of a very good team in a big stadium with a big tradition".
De Boer is the owner of the shortest reign as manager in Premier League history.
"I think it's his mentality", he said. 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. "But when you draw away even though it's a 0-0 - and that's not an especially good result - but we get to decide the tie at home, we don't have to win three, four, five, we have to win 5-4, 8-7, 2-1, 1-0, not to win the tie be disappointing". Not many were expecting him to start against Liverpool and he had a good match not just about scoring goals.
Jesse Lingard has praised Marcus Rashford's mentality following the Manchester United striker's match-winning display against Liverpool at the weekend.
You could see he wasn't right - everything which was easy for him previously became a difficulty and Rashford's natural game had vanished as a effect of not having a rest.