It's never a good idea to get into a war for words with Jose Mourinho.
The former Dutch worldwide had praised Rashford and said it was a "pity" that he plays for Mourinho, who only cares about results, and not a manager who would continue to play him even if he has a bad game or two.
Rashford scored twice in United's 2-1 win over Liverpool last weekend as his side extended their lead over Liverpool to four points in second place.
De Boer suggested Mourinho was not the right manager to nurture Rashford's talents, with Mourinho quick to pour scorn on the views of a coach who has failed in his last two jobs at Inter Milan and Palace.
"I read something, some quote from the worst manager in the history of the Premier League", the United boss said.
He said: "It's a pity that the manager is Mourinho because normally, he's an English player, you want to give him time and he can then make mistakes, but Mourinho is not like that". I would say he was in the top five with more matches for two season and applies to Scott the best thing to happen is to have experience.
The former Netherlands defender, who worked alongside Mourinho at Barcelona when the United manager was Louis van Gaal's assistant, didn't win any of his league games as Palace manager and his team didn't score a goal.
"If he was coached by Frank he would lose because he lost every game".
And after hearing the comments from de Boer, Mourinho did not hold back in his criticism of the former Palace manager.
The perception of United as a pragmatic team set up to get results continues to grate with Mourinho as he faces Sevilla following a goalless draw in Spain three weeks ago.
"I don't know (if he can play), we don't know yet, but he trained a little bit after you left". Does he play every match from the start? No?
"I try to give the best to the kid, I have to be honest and give the credit to the people of the academy that was responsible for his formation". "I don't think we're among the five candidates to win the competition. In the last eight you start smelling the semi-finals".
The Portuguese did not actually address that question ahead of the return leg against Sevilla, nor did he give a straight answer on Paul Pogba's availability on Tuesday. 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.