The 21-year-old failed to make the starting line-up for last weekend’s 4-0 win over West Ham but showed his quality after coming on with a well-taken goal and an assist for Paul Pogba.
Mourinho has been vocal in his desire to add a wide player to his squad this summer, with United strongly interested in Inter Milan’s Ivan Perisic.
But the United boss insisted that is not in an indictment of Martial, adding he was specifically looking for someone capable of playing wing back.
“I have faith in Anthony,” Mourinho insisted. “Probably the player I was looking at was not a pure winger because I have pure wingers.
“Probably the player that I was looking for is a player that allows me to play with three at the back and play wing backs.
“Because – as a pure attacking player – Anthony is a good player. He is one of the players I believe is going to have a better season this season than the previous season.
“I look at some of my players and I am sure that some players are going to be better. Anthony is that for sure.”