Martial, 22, left the club's tour of the United States last week.
"Matic arrived from the World Cup, holiday, direct to surgery because something happened in the World Cup", Mourinho revealed.
Martial's future with the club was already in question prior to this situation, having seen his playing time drop after Jose Mourinho took over for Louis Van Gaal following the 2015-16 season.
The club's inconsistent performances has led Mourinho to express his frustration on multiple occasions throughout the USA tour, including criticising individual players and United's lack of transfer activity.
Asked whether Martial would link up with United, Mourinho said: "Anthony Martial?".
For now, Mourinho can only focus on trying to get the players ready for the start of the season. "I know the one who are ready for anything, and I know the ones who are not ready".
Martial has been one of the most talked about players this summer in regards to a move, but rumoured links with Juventus, Bayern Munich, Paris Saint-Germain and even Tottenham haven't come to fruition.
United are set to be without a host of their stars for the club's Premier League opener against Leicester City on August 10.
According to the same story, the Manchester United board believe the root of Mourinho's unhappiness is absence of some key Red Devils players following their participation in the World Cup.
"With the fantastic spirit we have in the majority of the players - I repeat, in the majority of the players, we have a fantastic spirit - we go with everything we have to the Premier League".