Rashford has made 39 appearances for United across all competitions, but he has not started a Premier League game since Boxing Day due to the improved form of Anthony Martial and January arrival of Alexis Sanchez.
The 20-year-old is still regarded as one of the brightest prospects in English football, but Mourinho acknowledged that his blistering start to life in the senior side may have raised expectations too high at this stage of his career.
"At his age, what he's doing is more than enough and the experience he's getting at every level is more than enough for us to be happy with what we think is going to be his future. It's as simple as that," Mourinho told reporters.
"But because he had such an impact at the beginning, probably people expect him to play even more than he does and score even more than he does and perform even more than what he does but it is not so simple. What I see makes me really happy, to see the same boy.
"When you ask me about [Scott] McTominay, I spoke about McTominay as a boy before he was a player and Marcus is the same.
"What will keep them in the right direction, what will make them have that stability to improve is what they are as boys. And Marcus is a fantastic boy, also very grounded. For sure, we love him, and we believe in him, and he's going to have the chances."
Rashford has scored just four goals in 25 Premier League appearances this season, only 13 of which have been from the start.