This season, the England worldwide has now scored six times in the league, but he has mostly been used as a substitute.
Slightly critical of Mourinho, and you feel De Boer's criticism would be better aimed at the high pressure setting of a top four club as the reason for Rashford's stunted development.
"It's a pity that the manager is Mourinho because normally [Rashford's] an English player you want to give him time and he can then make mistakes", De Boer said while appearing as a pundit on BT Sport.
"'If he has one or two not good games, he puts him out", he added.
"If you go to his numbers and how many games he played with me".
The United boss had an acerbic response to hand when he addressed a news conference ahead of Tuesday's Champions League showdown with Sevilla, seemingly holding De Boer responsible for Palace's slump extending to seven winless and scoreless matches as Roy Hodgson picked up the pieces at Selhurst Park.
Win Over Hong Kong Takes Nepal Closer to ODI Status
Lamichhane finished with three for 17 from his 10 overs - fantastic figures that only strengthens his reputation as a future star. If Nepal can beat Hong Kong , by a huge margin, they can surpass Afghanistan to be the third team in Super Six.
It's never a good idea to get into a war for words with Jose Mourinho.
Frank De Boer's words come across as odd due to the reality of the situation as Jose's love for the youngster has been publicly stated multiple times.
"I think we're not one of the best teams in the competition but when a team arrives in the last eight, anything can happen", said Mourinho, who said Paul Pogba did appear for a "little bit" of training after missing the start on Monday morning but was unsure if the midfielder would be involved.
"If Marcus would be coached by Frank, he would learn how to lose".
'Marcus I have to be honest and give the credit to the people of the academy that were responsible for his formation and to Van Gaal for his first season in the first team'. I said that before the Liverpool match when many of you were maybe not expecting him to start. Of course not. He's a player we trust.
'I am really pleased with everything. His goals against Anderlecht and Celta Vigo ensured no player was more important in the Reds' run to the Europa League final. "I'm really happy with Marcus".