Casillas has fully recovered from a hand injury.
Karanka said: "He's evolved better this week and we're pleased to have him with us because we've got two decisive months ahead. He's ready to play and he's going to be called up. Having him with us is a joy and I don't have to evaluate what Sacchi said about how he trains.
"He's important for us, for the team and for everyone. The coaching staff wants the best for the team and he won't play tomorrow, but you never know what will happen in the future."