Calum Davenport is in hospital after fracturing a bone in his neck during his first match on loan at Watford.
Davenport had joined Watford with a view to a permanent move
The West Ham defender was stretchered off after team-mate Darius Henderson landed awkwardly on him during the 1-1 draw at home to Charlton Athletic.
The injury has triggered an immediate end to the 25-year-old's loan spell and he will return to the Hammers.
"Calum heard a crack in his neck and he lost the sensation in his fingers," said Watford manager Aidy Boothroyd.
"When it is a neck injury you have to get him off the pitch straight away and we hope it is not too bad."
Davenport is set to remain in hospital under observation for the next few days.
The Hornets, currently second in the Championship, are expected to move in the transfer market to bring in a replacement.
Davenport himself was signed as cover for Danny Shittu, who is away with the Nigeria squad at the African Nations Cup.