Medical staff taped up his ribs after the incident but it quickly became clear he would be unable to continue and he was replaced by Diego Lopez.
Casillas has barely played since Alvaro Arbeloa accidentally kicked his hand and broke a bone in January.
He fell out with former Real coach Jose Mourinho on his return and was replaced in the starting lineup by Lopez.
Mourinho's successor, Carlo Ancelotti, has used Lopez in Real's four La Liga matches this season but picked Casillas, who has retained his place in the Spain team, for their opening Champions League game.
Group I leaders Spain play their final two World Cup qualifiers at home to Belarus and Georgia on October 11 and October 15 respectively.
(Reporting by Iain Rogers, editing by Stephen Wood)
