Reigning Tour de France champion Jonas Vingegaard suffered a collapsed lung in Thursday’s crash in the Itzulia Basque Country race.
The 27-year-old Dane broke his collarbone and several ribs in a high-speed crash on a descent on stage four.
His team, Visma-Lease a Bike, said on Friday that hospital tests also revealed bruising or bleeding of the lung tissue.
They said: “He is stable and had a good night. He remains in hospital.”
Race leader Primoz Roglic and fellow Grand Tour winner Remco Evenepoel also abandoned the race after going down in the crash.
The remaining 35km of the stage were neutralised.
Wout van Aert, Vingegaard’s leading domestique and winner of nine stages of the Tour, broke his collarbone and several ribs in a crash at the Dwars door Vlaanderen one-day race on 27 March.
This year’s Tour de France starts on 29 June.
In Friday’s penultimate fifth stage, 21-year-old French cyclist Romain Gregoire, of Groupama-FDJ, took victory in a sprint finish from Orluis Aular.
Lidl-Trek rider Mattias Skjelmose, who took the overall lead on Thursday following the loss of several high-profile riders, saw his narrow lead reduced to two seconds by Max Schachmann, who crossed the line third.
It leaves the top 10 riders separated by 30 seconds heading into Saturday’s demanding 137.8km finale which starts and ends in Eibar.