Ralf Rangnick revealed Anthony Martial missed Manchester United's 2-2 draw against Aston Villa on Saturday after the unsettled France forward said he did not want to be in the squad.
Rangnick was forced to leave one place on the substitutes bench unfilled at Villa Park after Martial opted not to make the trip to Birmingham.
United's interim manager said the 26-year-old did not want to travel to Birmingham as he pushes to leave before the January transfer window closes.
Martial, linked with a loan move to Spanish club Sevilla, has not played for United since the 3-2 win over Arsenal on December 2 - the final game before Rangnick officially took charge at Old Trafford.
"Yes, he didn't want to be in the squad," Rangnick told reporters.
"He would have been in the squad normally but he didn't want to and that's the reason why he was not travelling with us yesterday."
Martial joined United in 2015 from Monaco but has struggled to hold down a regular first-team place in recent seasons.
He is under contract at United until 2024, with the club holding the option of an additional year.