England forward Rashford scored two wonderfully taken goals as he starred decisively in United's 2-1 Premier League win over bitter rivals Liverpool on Saturday night (AEDT) - his first top-flight start since Boxing Day.
Mourinho, who will lead United into the home leg of their UCL tie with Sevilla tomorrow, was hitting back a De Boer who said it was "a pity" for Marcus Rashford that Mourinho is his manager.
Rashford scored two brilliantly-taken goals and has surely put himself in contention to start against Sevilla in the Champions League. He's a player, he's very young, he needs games, but he's so talented.
Yet, some labelled him as a fringe player simply because Alexis Sanchez's arrival gave Mourinho the opportunity to rest a 20-year-old who had played nearly every game for United.
Manchester United, who are second in the Premier League table after Saturday's win, are now preparing for their crunch clash against Sevilla in the last 16 of the Champions League on Tuesday night.
"If he [Rashford] was coached by Frank, he would learn how to lose because he lost every game", Mourinho said. I try to give the best to the kid.
"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 is aware of such history surrounding United and as long as Rashford's performances remain at a high level, he will continue to get opportunities. He wants to get results. "When you're in the last 16 it looks really far but in the last eight you start smelling [the] semi-finals".
Paul Pogba returns to the squad after a minor injury, but the club's record signing must do with a place among the replacements.
Midfielder Michael Carrick, meanwhile, has announced his intention to retire at the end of the season. In training is where I'm doing most of my learning. "In football you are going to be up, you're going to be down".
It appears as though they have instantly forgotten that the youngster was the Portuguese boss's preferred option at the start of the season, but it was his own poor form, coupled with the hard work and brilliant form of Anthony Martial, that saw him dropped in the first place.
Carrick is set to join United's coaching staff in the summer.