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.
Mourinho also took aim at the club's fans after the Liverpool game as sections of United fans jeered midfielder Scott McTominay for trying to slow the game down and Mourinho for substituting Rashford with midfielder Marouane Fellaini.
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.
The United boss hit back by describing Frank as the "worst manager in Premier League history" following his failed spell with Crystal Palace at the start of the season, but Ronald has attempted to play down the spat before it gets too personal. "I would say he was in the top five with more matches for two seasons", Mourinho said.
"If he was coached by Frank he would lose because he lost every game".
The former Chelsea manager has not just talked the talked though but has also walked the walked as Marcus Rashford was the only player involved in every match-day squad prior to his injury. "No, of course not but he's a player we trust and I told him that before the Liverpool match", added Mourinho. I'm really happy too.
Mourinho's defensive tactics in a 0-0 draw in Spain three weeks ago were much-criticised as, without an away goal, United's tie with Sevilla is finely balanced.
"The result in the first leg leaves everything open, we don't feel we have any advantage, we are going to try to prove we are better but we don't have any advantage at all".
YOU can lambaste Jose Mourinho for his - at times - pragmatic approach to football or if you disagree with how he deals with the press but one thing you cannot question is his approach to Marcus Rashford. The best thing that can happen to a kid is experience and Marcus is having this'.