Man Utd midfielder Casemiro ruled out of Fulham clash with injury
The Brazilian international was replaced by Sofyan Amrabat at half-time in the match which saw his team eliminated from the knockout competition.
"Just before half-time, he got an injury. That’s why we had to take him off," ten Hag said without specifying the nature of the injury.
"We have to wait 24 hours, but I think definitely for Saturday he's out," he told a post-match press conference.
Casemiro, a key player for the club in defensive midfield, last month injured his ankle while representing Brazil in a World Cup qualifier against Venezuela.
He was deemed fit to play against Uruguay five days later, but the club chose to leave him out of a trip to Sheffield United to help him recover.
Casemiro's latest injury comes ahead of Brazil's mid-November World Cup qualifiers against Colombia and champions Argentina.
Ten Hag said he could not say whether Casemiro would be recovered by then.
Manchester United are eighth in the league after losing Sunday's derby to Manchester City, while Brazil are third in the World Cup qualifiers after October's draw with Venezuela and loss to Uruguay.
