"If Marcus would be coached by Frank, he would learn how to lose".
Despite featuring forty times for United this season, Rashford has struggled for starts with Saturday's derby against Liverpool being the striker's first of 2018.
According to 101 Great Goals, Frank said to BT Sport: "It's a pity that the manager is Mourinho".
"Mourinho is not like that, he wants to get results".
De Boer added: "He can make mistakes but Mourinho is not like that and if he has one or two not good games then he puts him out".
"Tomorrow is a very hard match, playing against a team that didn't lose against Liverpool home or away [in the group stage], that drew in Liverpool and I say Liverpool because they are an example of a very good team in a big stadium with a big tradition".
Of course, Mourinho was made aware of De Boer's comment at his Monday press conference, and the Portuguese was fairly merciless in his response. "He's very young. He needs games, he's so talented - you want to see this player every week".
Mourinho went on to credit United's academy staff and his predecessor Louis van Gaal for their role in Rashford's development and insisted the player remains in safe hands under his own leadership. The best thing that can happen to a kid is experience and Marcus is having this'. He plays for United many matches, he starts some comes from the bench for others.
"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". Not many were expecting him to start against Liverpool and he had a good match, not just about scoring goals. I'm really happy with Marcus'.