Jose Mourinho has branded Frank de Boer "the worst manager in the history of the Premier League".
The Manchester United boss took aim at De Boer in a press conference on Monday after the Dutchman had suggested Mourinho was not the right man to develop Marcus Rashford.
“I read a quote from the worst manager in the history of the Premier League , Frank De Boer – seven matches, seven losses, zero goals – that Marcus would not learn under me,” said Mourinho.
“If Marcus would be coached by Frank, he would learn how to lose.”
De Boer had said it was a "pity" Rashford was coached by Mourinho after the 20-year-old forward scored both goals in United's 2-1 win over Liverpool.
Sunday's clash was the first Premier League match Rashford - one of English football's brightest prospects - had started in 2018.
De Boer, who was sacked by Crystal Palace after four successive defeats at the start of the season , told BT Sport: "It's a pity that the manager is Mourinho.
"Normally he's an English player, you want him given time, he can make mistakes.
"Mourinho is not like that, he wants to get results. If he has one or two not good games, he puts him out."