Eighteen-year-old striker Rashford scored eight goals for United last season having only made his first-team debut in February and was rewarded with a surprise spot in Roy Hodgson’s England squad.
After netting on his international debut in a friendly against Australia, he was then limited to two late substitute appearances at Euro 2016, where England crashed out to Iceland in the last 16.
Hodgson has been replaced by Sam Allardyce and Rashford was named in Gareth Southgate’s England Under-21 squad this week.
Mourinho said on Friday: “I prefer it this way, because to be selected (for the seniors) and be on the bench behind (Harry) Kane and (Jamie) Vardy and all the experienced players, and play one minute or not play…
“I would prefer him to go to the Under 21s and play what Southgate decides, but I believe 90 or 80 minutes, a big part of matches, and contribute to his development.”
Rashford ended up making 18 appearances – all of them starts – last season under Mourinho’s predecessor Louis van Gaal.
This term his only action so far has been as a substitute in the Community Shield, while new frontman Zlatan Ibrahimovic has come in and netted four goals in three starts.
But Mourinho insists Rashford, who he has described as “amazing” for his age, has a bright future at United and will play “a lot”.
The Portuguese said: “Marcus is a young and talented player. We bought one striker (Ibrahimovic) – not two or three or four. If you don’t trust him or want him to play, you buy other strikers. That is not the case.
“Zlatan is playing and is showing why, and Marcus is our striker number two at the moment. For an 18-year-old boy, I think he is amazing.”
And Mourinho added: “They can play together because they did it already. They played against Leicester (the Community Shield game, a 2-1 victory) as a two when Marcus came on.”