Despite featuring forty times for United this season, Rashford has struggled for starts with Saturday's derby against Liverpool being the striker's first of 2018.
De Boer praised Rashford after the match and said it is a "pity" the 20-year-old plays under Mourinho at Old Trafford - prompting an angry response from the United boss on Monday.
"I'm not upset with the fans about Rashford", Mourinho said after the match.
Saturday's victory was the Portuguese manager's 68th Premier League game as United boss and Rashford has played a part in 59 of them.More news: Mystery $560M Powerball victor can remain a mystery, judge rules
More news: Who will Charlotte Flair be facing at WrestleMania?
More news: Glitch At Fertility Clinic Exposes Vulnerabilities In System For Women Freezing Eggs
Mourinho was referencing De Boer's stint with Crystal Palace at the start of the season in which he took charge of seven PL games winning none, losing all and scoring no goals in the process. "In training is where I'm doing most of my learning", he said. Of course, with Marcus, I have to be honest and give the credit to the people in the academy who were responsible for his formation; I have to leave credit to Louis van Gaal, who was responsible for his first season in the first team.
Mourinho used his press conference on Tuesday ahead of their Champions League round of 16 clash with Sevilla, to fire back at the Dutchman.
"But if you go to his numbers and how many matches he played with me last season and this season, I would say probably he's in the top five players with more matches in the two seasons and the same applies to Scott McTominay".
"The best thing that can happen for a kid is to have experience and Marcus is having these experiences". I am really pleased with everything. There are only 11 players that can start - it's just one of the those things.
The La Liga club held Liverpool 2-2 at Anfield in the groups before coming from behind to draw 3-3 at home, and are more than capable of getting the away goal that could prove fatal to United's progress. I'm really happy with Marcus'.