Mourinho defended his use of the 20-year-old during a news conference on Friday, even with the England international suspended for United's trip to Watford .
Questions have been asked over Rashford's best position, with the forward used centrally and out wide at the club level.
Mourinho believes Rashford is capable of performing in any role in a front three, but says his wide players had the freedom to move inside rather than playing as true wingers.
"To be in a system of four, to be a winger, it takes him a little bit out of his positions, but the way Manchester United plays is with three attacking players," he said.
"When we play with three up top – because sometimes we also play with two – our three attacking players are not wingers. They're not wide or open all the time.
"They have the freedom to come and play in between positions, and any one of the three positions is a good position for him."
Mourinho said Rashford had to be used centrally by England due to Gareth Southgate's system.
"I think his position is in relation to the way his team plays. So I think in the case of the national team, they play normally with five at the back and two players in attack. I think that's his position," he said.
"He cannot be a wing-back in the system of five – he cannot replace Luke Shaw, Kieran Trippier or Danny Rose at all. He must play centrally in that system."