Winston Reid will miss the rest of the season after suffering an injury whilst playing against Swansea.
The 29-year-old West Ham defender was taken off on a stretcher with an oxygen mask over his face after being knocked unconscious and twisting a knee. His side went on to lose the match 4-1.
Full-back Sam Byram, 24, could also miss the rest of the campaign after injuring his ankle in the same game.
David Moyes' team are currently 14th in the Premier League, three points above the relegation places.