Reports in France this week claimed Martial had chose to leave Manchester after being frustrated with a lack of playing time.
The United boss said the number of the England international's appearances from the bench are "because I decide that way", before brusquely downplaying the suggestion that it was hard to keep the squad happy.
The former Chelsea boss said he can not guarantee playing time to either player. It's just football. "We can not have 11 players [in the entire squad]".
When Martial not being first-choice was brought up, Mourinho explained: "If I was you I would go with why [Romelu] Lukaku plays every match, why [Nemanja] Matic plays every match".
Only 11 can start so the same questions you are doing to me, you should do to every other manager because everyone has players in the same situation.
Martial, meanwhile, has started 16 Premier League games but none since February, although he has been sidelined due to injury, with both forwards hoping to be selected for the World Cup. You can not have just 11 players.
United face West Brom on Sunday before travelling to Bournemouth and it could be good opportunity for Mourinho to hand some fringe players game time.
"If I was you, I would go in another direction".
When asked about West Brom's position, Mourinho admitted: "I am surprised".
"I think it is a big challenge because they know that in football anything is possible until mathematically impossible".
"We need the points and obviously to finish fourth we need four or five points, but to finish second we need much more than that".