Casillas is close to returning from injury, but Karanka did not confirm whether the goalkeeper, who is Spain's record cap holder with 143 appearances, will return to the starting lineup.
"San Mames is special for any player," said Karanka.
"For Iker, for me, for Xabi Alonso, for Sergio Ramos. We still don't know who will play."
Real Madrid were drawn against German side Borussia Dortmund in yesterday's Champions League semi-final draw.