The 20-year-old has not started a Premier League game since Boxing Day.
But Mourinho said Rashford is a key player at United and always “starts or is on the bench”.
“If the national coach trusts him, he selects him. It doesn’t matter if he plays or doesn’t play,” he said.
Rashford has won 15 international caps and scored on his debut in May 2016 but only has two goals in total for England.
Despite his lack of recent starts Rashford has played 38 times for United this season in all competitions, including substitute appearances, and has scored 10 goals.
Mourinho played down concerns about him being selected for the World Cup in Russia.
“I don’t reassure anyone,” he said. “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.
“I see it sometimes put in doubt if he is going to be selected for the World Cup.
“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.
“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.”