De Boer has previously commented on the development of United forward Marcus Rashford, with the Dutchman claiming that Mourinho cares more about results than the progression of the younger members of his first-team squad.
However, after Rashford returned to the starting lineup to score twice against Liverpool at the weekend, Mourinho has moved to respond to comments made by De Boer, who lost all four of his Premier League fixtures in charge of Crystal Palace at the start of the season.
At a press conference, the 55-year-old told reporters: "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.
"If Marcus would be coached by Frank, he would learn how to lose."
Rashford has struggled for starts in the Premier League since the turn of the year but in total, the 20-year-old has featured in the first XI on 21 occasions in all competitions this season.