The centre-back was forced off with injury late in United’s 2-2 draw in the Europa League with Sevilla and will now miss the rest of the season.
“I can’t say in this moment, when we have news we will give it to you,” Ten Hag told reporters on Friday.
“Let’s do the medical, a full assessment, complete diagnosis, it doesn’t change in this moment. I have an idea but I don’t [want to] cause speculation.”
United are also without Raphael Varane, who came off at half-time against Sevilla with an injury that Ten Hag revealed he has been dealing with for a couple of weeks.
It means Harry Maguire and Victor Lindelof are set to be the centre-back pairing at the City Ground, though the United boss raised the possibility that Luke Shaw could return to action this weekend.
The left-back, who has filled in in the centre of defence this season, has been out since picking up an injury against Brentford, but Ten Hag confirmed that both he and Scott McTominay will be assessed over the next couple of days and could be involved against Forest.
Marcus Rashford remains out with a groin injury, and Wout Weghorst may well be given the nod to lead the line in his absence. Anthony Martial started against Sevilla and impressed, but United are keen to avoid risking any further setbacks for the Frenchman.
“Maybe he can [start again] but it’s a risk and we can’t take that risk in this moment because we don’t have so many options in the frontline with Rashford and [Alejandro] Garnacho injured, so we have to be careful,” Ten Hag said.
”We know when there’s a lot of load he gets injured and we have to build it, especially when the games come quick.”
Casemiro is back from suspension and should feature in the Premier League for the first time in more than a month, while Christian Eriksen is also pushing for a start after two appearances off the bench against Everton and Sevilla.
Predicted Manchester United XI: De Gea, Wan-Bissaka, Lindelof, Maguire, Malacia, Casemiro, Eriksen, Fernandes, Antony, Weghorst, Sancho