The Brazilian was absent as United lost 3-0 to City in the Manchester derby at the weekend, but returned to the line-up as United were dumped out of the Carabao Cup after losing by the same score to Newcastle.
Speaking in his post-match press conference, Ten Hag confirmed he would not be available when United travel to Craven Cottage for the early kick-off on Saturday.
'He was injured,' the Dutchman said.
Casemiro will miss Manchester United's trip to Fulham on Saturday afternoon
Erik ten Hag said the midfielder was injured in the first half of the 3-0 loss to Newcastle in the Carabao Cup on Wednesday night
'Just before half-time, he got an injury. That’s why we had to take him off.'
Asked whether the midfielder may be ruled out until the international break in mid-November, he replied: 'I can’t give an answer on that [now].
'We have to wait 24 hours, but I think definitely for Saturday, he is out.'
Casemiro's absence comes at a delicate time for United, with pressure mounting on Ten Hag after a dismal start to the season and a difficult run of games coming up.
After the trip to Fulham, United travel to Copenhagen for a pivotal Champions League fixture, followed by a home against Luton.
Following the international break, Ten Hag's men face trips to Everton, Galatasaray and Newcastle in a week.
Casemiro was a standout performer in his first season at Old Trafford following his £70million move from Real Madrid, scoring in the Carabao Cup final against Newcastle.
The 31-year-old has failed to replicate the form this term and has regressed along with the rest of the team.
The Brazil international has missed United's last two Premier League games but remains the club's top scorer this season with four goals in all competitions.
The Brazilian midfielder has failed to hit the heights of last season and has struggled this term
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