The 31-year-old midfielder has struggled for form and fitness in his second season since his eye-catching switch to Old Trafford from Real Madrid.
Casemiro sustained an ankle injury on Brazil duty last month and returned against Newcastle last Wednesday, only to be taken off at half-time of the 3-0 Carabao Cup defeat.
United confirmed the Brazil international had sustained a hamstring that would keep him out for “several weeks”, but Ten Hag has now suggested the midfielder may struggle to play again this year.
“I have some time schedules, yeah,” Ten Hag said. “But it’s always difficult to project that because there can always be setbacks during the rehab period.
“I’m always a little bit holding back on such issues.
“I expect some back but, for instance, Casemiro and Martinez are really strong injuries and I don’t expect them back before Christmas.”
Among the 10 matches Casemiro is expected to miss before Christmas is Wednesday’s key Champions League group game at Copenhagen.
The latter has enjoyed a resurgence of late and Brain injury charity Headway questioned the decision to allow him to continue playing after a head collision in Saturday’s 1-0 win at Fulham.
Maguire completed the match and is in United’s 23-man squad to face the Danish champions.
“So, we did all the protocols during and after the game, so he is ready to play,” Ten Hag said. “There is not any indication (of concussion).
“He was fine in the game so no problems. He played a very good game.”
United’s squad also includes Victor Lindelof and Marcus Rashford, who missed the trip to Craven Cottage through illness and a leg issue respectively.
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“100 per cent (fit),” the United boss said. “Rashy missed one game out. A small knock but he’s back.”
United will hope the pair’s return can help continue their revival after making hard work of beating Copenhagen on home soil, having lost their Group A openers against Bayern Munich and Galatasaray.
“They played a very decent game and I thought we didn’t play a good game,” Ten Hag added.
“But, still, we won. I think it was a deserved win but we have to do things better.
“We are aware of that because you have to lift the performance if you want to get the result in.”