Tottenham defender Jonathan Woodgate spent Saturday night in hospital.
He left the field 10 minutes into the 0-0 draw with Manchester United at White Hart Lane.
Woodgate was taken to hospital where doctors tried to discover the cause of the problem before they discharged him yesterday.
Assistant Spurs boss Kevin Bond said: "He was in hospital with a back spasm but they're not sure of the prognosis."
Spurs will be hoping the injury is not the same back problem that hindered his progress at Real Madrid.
Woodgate, 28, is almost certain to miss Thursday's home UEFA Cup clash against Spartak Moscow.
Jermaine Jenas is also likely to miss out, while Ledley King (knee) cannot play in every game.
Spurs boss Harry Redknapp said: "I've got kids out of the youth team because you have to have so many homegrown players in your squad."