Mourinho, clearly angered by De Boer's comments, fired back at the ex-Netherlands star by branding him the Premier League's worst manager of all-time.
"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. "But when you draw away even though it's a 0-0 - and that's not an especially good result - but we get to decide the tie at home, we don't have to win three, four, five, we have to win 5-4, 8-7, 2-1, 1-0, not to win the tie be disappointing". "In the last eight you start smelling the semi-finals".
"He was saying it is not good for Marcus Rashford to have to be coached by me".
"But if you go to his numbers and how many matches he played with me last season, I would say probably he's in the top five players with more matches in the two seasons I've been here".
De Boer is a protege of Mourinho's predecessor and ally turned enemy, Louis van Gaal, and while Mourinho gave credit to the older man for initially selecting Rashford, the days when they worked together at Barcelona seem distant indeed.
JOSE MOURINHO has defended his kid-glove treatment of Marcus Rashford. But when a team arrives in the last eight, anything can happen. He plays for United many matches, he starts some comes from the bench for others. He's a player we trust. I'm really happy too. I said that before the Liverpool match when many of you were maybe not expecting him to start.
On a yellow card, Marcus was substituted midway through the second half and, when asked about that decision to make the change, Jose told reporters: "I heard Gary Neville was asking [on Sky Sports] for Marcus Rashford to get a red card and I was scared maybe the referee was watching it at half-time!"
"I don't think we're candidates to win the competition".