The Argentina international, who was out for four months after having surgery on a foot problem, hobbled off in United’s 3-0 win over West Ham after hurting himself in a challenge with Vladimir Coufal.
Ten Hag is waiting to discover how long the centre-back faces on the sidelines but is bracing himself for the worst after declaring it looked “very bad”.
He said: “There is a big concern. We have to wait for a few days for the diagnosis and see what is the damage. He is very sad, very disappointed. We all feel really with him. First of all, it is a personal disaster when it is really bad but also for the team it is really bad because he definitely brings us a lot.
“It is very bad for him, very bad for the team. In this moment we are very sad, hopefully it is not too bad but we can only pray. It looks like he has to sit some games out injured again. He is very important for us. He brings spirit to the team, aside from his football ability. We can only pray.”
Rasmus Hojlund and Alejandro Garnacho, with two, struck against West Ham and Ten Hag said: “It was not easy, West Ham are a good side, but it was a great win for us.”