De Boer lost all four of his Premier League games without scoring a goal before he was sacked by Palace in September.
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.
"If he was coached by Frank he would be coached how to lose because he lost every game".
"I have to be honest and give the credit to the people of the academy that was responsible for his formation, to [former United boss] Louis van Gaal, who was responsible for his first season".
Mourinho includes Pogba in United's Champions League squad to face Sevilla
Man United players during a training session on the eve of their Champions League Last-16 tie against Sevilla in Manchester yesterday.
"He's very young, he needs games".
"He was saying that it is not good for Marcus Rashford to have a coach like me, because the most important thing for me is to win".
"Marcus is having these experiences. Analysing a performance for me is more than number of goals or assists, and for me I'm really happy with Marcus". He plays for United many matches, he starts some comes from the bench for others. "I'm really happy with Marcus".
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"Our People in Albuquerque worked to get Customers on board another aircraft to continue their journey to Dallas". Breathing heavily, he called his wife back home in North Texas to tell her what had happened.
"I think that Gareth Southgate doesn't mind that I say a couple of minutes of our conversation were about Marcus", Mourinho said.
De Boer had claimed that Mourinho did not have the patience to let Rashford develop because he was all about getting results.
Later that evening, on BT Sport show Premier League Tonight, De Boer said it was a "pity" that Rashford was playing under Mourinho, that the 20-year-old would receive more playing-time under a different manager, as Mourinho's only focus is gaining a result.
"A disaster?" he said."We won 2-1 against Liverpool and there were some who said it was a disaster". We have a game on Tuesday and I can imagine, if we lose against Sevilla, everybody will forget what we did in the past couple of weeks. But when a team arrives in the last eight, anything can happen. "I don't feel we have any advantage in this moment".
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I can't keep doing this because I am letting the team down and I am letting myself down", Rabada said. I became a dad at the same time, all of a sudden two boys are around and I've got a family.
Mourinho confirmed Paul Pogba returned to training on Monday afternoon but admits he is unsure if the midfielder will be able to play a role against Sevilla after missing the win over Liverpool.