Martinez's knee buckled under the weight of West Ham defender Vladimir Coufal in the 66th minute of the Reds' 3-0 Premier League win at Old Trafford and he was forced off in the 71st minute.
Ten Hag was unable to provide a specific prognosis on the injury but said it was 'very bad' and Martinez was 'very sad! "I can't say, it doesn't look good so there's a big concern, but we have to wait for the coming days what the diagnosis is then we can tell you more.
"We do everything to get the right diagnosis to see what the damage is. He is very sad, very disappointed. We all feel really with him. First of all, it's a personal disaster when it's really bad. But let's wait for what it is.
"But for the team, also it is really bad because he definitely brings us a lot." Martinez returned from a four-month lay-off with a foot three injury against Tottenham weeks ago and he started in the recent wins at Newport County and Wolves.
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