De Boer said after Rashford had scored twice for Manchester United against Liverpool on Saturday that it was "a pity" Mourinho was his manager because he lacks patience with youngsters and is too interested in results.
"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".
"If you go to his numbers and how many matches he played with me last season and this season, maybe I am wrong, but I would say he is probably in the top five players with the most matches over two seasons".
Rashford fired two goals as United secured a 2-1 win over Liverpool to cement their hold on second place in the Premier League, prompting ex-Crystal Palace boss De Boer to question why he doesn't feature more regularly at Old Trafford.
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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".
"But if he was coached by Frank he would lose because he lost every game".
The 20-year-old staked his claim for a spot in the team with a clinical brace against their neighbours rivals, but in the run up to the game Frank De Boer argued that Rashford's development was being halted at his current club.
Furthermore, the signing of Romelu Lukaku has seen the Englishman deployed as a winger on numerous occasions, and De Boer does not believe Mourinho has the patience to allow Rashford the necessary room to develop.
"I try to give the best to the kid", said Mourinho. He plays for United many matches, he starts some comes from the bench for others. He's a player we trust.
"He had a good match and not just by scoring goals". I'm really happy with Marcus'. Does he play every match from the start? No?