Asked about Rashford ahead of United's Champions League clash with Sevilla on Tuesday night, Mourinho said: 'I read something, some quote from the worst manager in the history of the Premier League - Frank De Boer.
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.
De Boer, who has also managed Ajax and Inter Milan, suggested on Saturday that Rashford should play more often.
It's absolutely brutal from Mourinho but it's actually impossible to argue with his assertion that the former Inter Milan manager is the worst manager that the Premier League has seen, statistically at least.
De Boer was sacked as Crystal Palace manager this season after losing all of his opening four league games.
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After all the discussion of whether Rashford is fading away under Mourinho at Man United, the Special One threw him a start against Liverpool and he kindly paid him back with a brilliant brace to win the staunch rivalry. If he was coached by Frank, he would learn to lose because he lost every game.
Rashford scored two first-half goals to help United to a 2-1 victory over their rivals for second place in the Premier League.
"He started, he had a good match and I'm not just speaking about scoring goals because, for me, the analysis of a performance is much more than analysing the number of goals or assists", said Jose Mourinho, as reported by the club's website.
"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".
"It was a great win and the Palace game, to get the result how we did shows what kind of team that we are and if we can get past Sevilla, it will put us in a good position and then we have the FA Cup".
"Mourinho has bought (Alexis) Sanchez in January and of course it's the truth that it's not good for Rashford's development and he will play less now for Man United".