During their FA Cup win over Stevenage, the Spurs skipper had to be carried off on a stretcher with nine minutes gone after falling awkwardly following an aerial challenge with Chris Beardsley.
Redknapp expects the centre-half to be out for the season.
"He has gone off to hospital. It wasn't good. It is his ankle and knee. Looking at his injury it would be a miracle to think he is not (out for the season)," said the Tottenham boss, who also had to substitute Aaron Lennon in the first half.
"Aaron did a hamstring," Redknapp added. "He keeps tweaking them when he plays."