The midfielder missed the 2-1 win over Sheffield United because of an ankle injury picked up while playing for Brazil during the international break and was suspended for the 1-0 win over FC Copenhagen on Tuesday.
He had been expected to return against City at Old Trafford but Ten Hag said the 31-year-old remains a doubt.
“Case [Casemiro] is a race against the clock,” Ten Hag said. “He’s not 100%.”
Alejandro Garnacho will be available to face City despite the FA launching an investigation into a social media post which contained two gorilla emojis above a picture of André Onana.
Onana has defended his teammate but Garnacho could still face a ban if he’s charged by the FA for a breach of their rules.
“We are talking with the FA,” Ten Hag said.
“I want to emphasise that we are together, we are united. You saw that with the post of Andre.”
United head into the game against City on the back of three wins in a row, although there remain question marks over their performances.
Ten Hag was critical of the display in the first half against Copenhagen but he insists his team are making progress.
“This is a hard team to beat,” Ten Hag added.
“They find a way to win, we’re going in the right direction. We have a way to go, I see positives and I see also negatives.
“It’s a match [against City] that is so huge because it’s Manchester, it’s internal, but it’s also about world football. All the eyes will be on this game globally.”