Rashford has started just 30 of the 70 Premier League games in which Mourinho has been in charge in the Portuguese manager's near-two seasons at the club.
Meanwhile Martial appears to have dropped down United's pecking order since the arrival of Alex Sanchez in the January transfer window, having failed to complete a full 90 minutes in the Premier League since the middle of February.
That has led to speculation that either, or both, could seek a future away from the club and, while Mourinho refused to comment on that prospect, it is understood United are not keen to listen to offers for either player.
However, Mourinho could do little to console the pair about their limited opportunities when he spoke at a news conference on Friday.
"It is just football, we cannot have just 11 players," he said. "It would be amazing economically for the football clubs to have only 11 and play with the same 11, no injuries, no suspension, no fatigue," Mourinho said.
'Nothing I can do'
"It would be better for everyone to have only 11 but is impossible. At every club and every level, from the top of the world to the amateur football clubs, you need more than 11 players and only 11 can start. It is as simple as that, there's nothing I can do.
"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."
It is a strange statistical anomaly that Rashford has actually featured in more first-team games than any other player in Mourinho’s time in charge of United -- a fact the manager is fond of reciting.
However, many of those outings have been off the bench or in competitions such as the League Cup or, last season, the Europa League.
Mourinho was in no mood to discuss their medium-term prospects but he did offer the cases of Romelu Lukaku and Nemanja Matic -- automatic first-team starters under the manager -- as examples of players Rashford and Martial should try to emulate.
The United boss has made no secret of his admiration for Lukaku and Matic, often singling them out for praise even while criticising other squad members.
"If I was you I would go ask why Lukaku plays every match, why Matic plays every match," Mourinho told reporters.
"You always go to that one. The answer is if you want what I call dry answers, I have dry answers. And a dry answer is only 11 can start the match and we are 22."
The situation concerning Lukaku is particularly interesting with the Belgian international now the only specialist centre-forward on United's books, following Zlatan Ibrahimovic's move to the United States.
At one stage, it had appeared that Martial could be a possible back-up to Lukaku although Mourinho has long since abandoned that experiment.
Sanchez remains a possible focal point for Mourinho's attack and the manager mockingly suggested he has no intention of trying to sign another forward in the summer transfer window to offer Lukaku some breathing space.
"No, no," said Mourinho," whose second-placed side face bottom-of-the-table West Bromwich Albion at Old Trafford on Sunday.
"If I have more competition for him I have more competition for the other ones you are saying are not happy because they are not playing.
"I probably have to reduce and have a squad of 12 or 13 players," he added sarcastically.