Manchester United have just one fit senior central defender for Saturday's Premier League game with West Ham.
Marcos Rojo is the only specialist option available to manager Louis van Gaal, with other players injured or suspended.
The United boss says he has never known anything like it in his time in management.
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"I have to admit, I've never been in this situation before. It's difficult to understand because we are evaluating what we can do in a different way," said the Dutchman.