The Real Madrid goalkeeper was in conversation with teammate Jan Vertonghen and issued the savage put-down.
As part of a game the two were playing, former Tottenham defender Vertonghen asked Courtois to name three "major" players who did not participate at the European Championship this summer.
But when the 29-year-old couldn't name anyone else, Vertonghen intervened and mentioned Manchester United forward Martial in his list.
Courtois wasn't having it whatsoever, though, commenting: "He's not a major player."
Courtois doesn't rate Martial at all:sob::sob::sob:
- Callum (@Chelseatransfe2) July 9, 2021
Martial has been capped 27 times by France, scoring one goal. After a disappointing season during which he scored just four Premier League goals, Martial was never in contention for Didier Deschamps' Euros squad.
The likes of Marcus Thuram, Olivier Giroud and Wissam Ben Yedder were ahead of him in the pecking order, while Karim Benzema received a shock recall and was paired in attack with Kylian Mbappe and Antoine Griezmann.
World Cup holders France were knocked out by Switzerland in the round of 16 after a dramatic 3-3 draw and subsequent penalty shoot-out.
Martial's future at United appears to be uncertain at this time following the impending capture of Borussia Dortmund star Jadon Sancho.
The Red Devils finally agreed a £73 million fee with Dortmund and are set to complete the deal after the Euros.
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