Frank de Boer is the "worst manager in Premier League history" and Marcus Rashford would "lose every game" if he played under the Dutchman, says Manchester United boss Jose Mourinho.
The Portuguese criticised De Boer, who had said on BT Sport that "it is a pity" Rashford plays under Mourinho.
De Boer was sacked as Crystal Palace manager this season after losing all of his opening four league games.
"I try to give the best to the kid," said Mourinho.
England international Rashford scored twice for United in their 2-1 victory over Liverpool on Saturday, having made his first start since Boxing Day.
It took his tally to 13 goals in 40 games in all competitions for the Red Devils this season.
Speaking on BT Sport's highlights show on Saturday evening, De Boer had said: "It is a pity that the manager is Mourinho. Rashford is an English player and you want to give him time and he may make mistakes, but Mourinho is not like that, he wants results.
"If he has one or two not good games, he puts him out. Marcus is very young and needs games, but he is so talented and you want to see him playing every week."
De Boer was in charge of Palace for only five games, winning one match in the EFL Cup, but the Eagles failed to score or collect a point in the shortest reign for a Premier League manager.
Mourinho said: "I read something, some quote from the worst manager in the history of the Premier League, Frank de Boer, saying it was not good for Rashford to have a coach like me because the most important thing for me is to win.
"If he was coached by Frank he would lose because he lost every game. 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, to [former United boss] Louis van Gaal, who was responsible for his first season.
"But if you go to his numbers and how many matches he played with me last season and this, I would say probably he's in the top five players with more matches in the two seasons."