"I don't think he lost his focus. I just think he didn't catch with both hands a big opportunity he had," Mourinho told British media.
United recently sold Memphis Depay to French side Olympique Lyonnais and Mourinho said Martial must make the most of his chance to nail down a first-team spot.
"When we spoke about Depay (leaving United) we spoke about it being the only position where we have an over-booking of players," the 54-year-old Mourinho added.
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"We have still five players for this position. I cannot give one player chances to play and kill the others.
"We have (Marcus) Rashford, (Jesse) Lingard, (Juan) Mata, (Henrikh) Mkhitaryan and Martial, and I cannot give the same player chance after chance after chance and not consider the effort of the others."
United, who head into Thursday's League Cup semi-final second leg against Hull City with a 2-0 lead, host second tier side Wigan Athletic in the FA Cup on Sunday before they face Hull in the Premier League three days later.
(Reporting by Shravanth Vijayakumar in Bengaluru)
