De Boer, who lasted only 77 days as Crystal Palace manager after losing his first four league games in charge with no goals scored, said in an interview on BT Sport that it was "a pity" Mourinho was Rashford's manager because he was not getting enough chances.
It's absolutely brutal from Mourinho but it's actually impossible to argue with his assertion that the former Inter Milan manager is the worst manager that the Premier League has seen, statistically at least.
I read something, some quote from the worst manager in the history of the Premier League, Frank De Boer.
'If he has one or two not good games, he puts him out.
United's focus now turns to their crucial UEFA Champions League last-16 second leg clash with Sevilla at Old Trafford on Tuesday.
It is little surprise De Boer's criticism has seemingly rankled with Mourinho.
"Seven matches, seven defeats, zero goals, what he was saying was it was not good for Marcus Rashford to have a coach like me because the most important thing for me is to win". If he was coached by Frank, he would learn to lose because he lost every game.
And a crucial aspect of Rashford's immediate impact came from the fact Mourinho took him out of the spotlight for some time.
His wonderful brace against Liverpool might have surprised non-Manchester United fans given it was his first start in the Premier League in 2018 but it didn't surprise the Old Trafford faithful.
You could see he wasn't right - everything which was easy for him previously became a difficulty and Rashford's natural game had vanished as a effect of not having a rest.
"I try to give the best to the kid".
"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".
As it turns out, the 20-yea-old has made more appearances during Mourinho's reign at United than any other player.