Mourinho unimpressed with Martial work rate
United produced another poor performance and were deservedly beaten at the London Stadium, with Felipe Anderson's strike, a Victor Lindelof own goal and Marko Arnautovic's effort doing the damage.
Marcus Rashford, a second-half substitute, got United's goal, but they slumped to another loss.
The defeat capped off a wretched week for United, who were dumped out of the EFL Cup by Championship side Derby County and saw a training-ground exchange between Mourinho and Paul Pogba create headlines.
Martial was one of the few to come out of the Derby defeat with any kind of praise, but he was disappointing against the Hammers.
A tough afternoon in the capital. #WHUMUN #MUFC pic.twitter.com/JG04kI4ra4
— Manchester United (@ManUtd) September 29, 2018
"I want to play Martial, something that you [the media] are asking for a long, long, long time," Mourinho said in his post-match news conference, which ended after just three questions.
"I left Alexis [Sanchez] out and I think Martial is not a player very, very focused on his defensive duties.
"I thought to play him as a second striker would be easier for him and at the same time I feel we need quality on ball in the building up from the back and Scott McTominay has that quality with the ball."
McTominay was making his first Premier League start of the season, having impressed Mourinho last term, and the 21-year-old midfielder was commended again by his manager.
"He is a special character," Mourinho said. "He is a special personality and a team in a negative moment needs this kind of mentality that Scott McTominay has.
"Not all of them, but every player is different, every player is a different person. Scott McTominay is a kid with a special character. He is young, but he is brave, so he is a special kid."