After struggling in Mourinho's first season in charge at Old Trafford last season, Martial's role has grown more influential this term, starting ahead of Marcus Rashford in United's last four games.
The France international scored his eighth goal of the season – matching his tally from the 2016-17 campaign – against Watford at Vicarage Road on Tuesday (28 November), tormenting the home side before being replaced after 65 minutes.
Despite his impressive return, which also includes six assists, the former Monaco sensation has been subbed off in seven of the 11 Premier League matches he has started this season. And heading into the busy festive period, his manager feels the 21-year-old is feeling the effects of an already-demanding campaign, explaining those concerns led to him being taken off against the Hornets.
"Yes, I am pleased. I think he suffers a little bit with accumulation of matches", Mourinho said after the 4-2 win in Hertfordshire. "I could feel that in the second half he was going down and down and down. He's still a young guy and his body, his biological situation has space to improve but in the first half, during the time, he had strength and explosion in his legs. He was dangerous and [scored] an important goal."
United will play nine matches in the month of December with fatigue a real possibility for a United squad that has already been stretched by injuries this season. Heading into the trip to the Emirates on Saturday (2 December), United could be without two key midfielders in Nemanja Matic and Marouane Fellaini. Matic was replaced by Ander Herrera after 54 minutes on Tuesday night, visibly limping as he left the pitch. Fellaini, meanwhile, was omitted from the match-day squad altogether with Mourinho revealing ahead of kick-off an injury meant he was not in contention.
Following their trip to north London, United's next Premier League fixture is against neighbours Manchester City at Old Trafford on 10 December.