Rashford has made 38 appearances for United this season, with 18 of them coming off the bench amid intense competition for places – something the arrival of Alexis Sanchez in January added to.
Mourinho thinks suggestions that could deny the 20-year-old, whose last Premier League start came in the 2-2 draw against Burnley on Boxing Day, a place in Gareth Southgate's squad for the competition in Russia are unwarranted.
"I see you sometimes put in doubt if he is going to be selected for the World Cup or not be selected," said Mourinho.
"You know, if the national coach trusts him, he selects him. It doesn't matter if he plays or doesn't play.
"There are many examples of players who don't play for their clubs at all and they go to the national teams. You have the example of [Sergio] Romero, who is the second goalkeeper at Manchester United and the first goalkeeper for such an amazing football country like Argentina.
"In your own country, you have examples of players who play even without scoring a goal in the Premier League.
"So it's up to Gareth Southgate. If he trusts him, he selects him. It doesn't matter if he plays or if he doesn't play for Manchester United."
Mourinho praised Rashford's attitude and pointed to his regular involvement in the squad – he was absent from a matchday squad for the first time this season at Newcastle United on February 11 – as sufficient to show what he is capable of.
"I don't reassure anyone. The main reassurance for him is that he is always selected. There is not one single match when Marcus is not selected to start or to be on the bench," said the Portuguese.
"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.
"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.
"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."