Even though Adán saw a red card against Real Sociedad, Real Madrid's second goalkeeper could start this game. It doesn't seem very likely, though, as the team is facing a very important match tonight in which they could get themselves eliminated from one of the two title options remaining. It's not that Casillas is in great shape neither (look at Real Sociedad's third goal), but he still seems to have the edge over Adán for these kind of games, especially after the Bernabéu clearly decided to support Real Madrid's captain.
Mourinho doesn't care much about the crowd's support. But imagine what would happen if Real Madrid conceded a couple of goals with Casillas on the bench in such an important game.
Marcelo isn't fit for this game yet, so he's not on the team's list. He could've helped a lot, but it might be better for Mourinho and the coaching staff not to rush him. That way, Arbeloa will probably be the team's left back and Essien will be on the right back position. The Bison (as they call him on Chelsea land), performed really well against Real Sociedad, but his lack of speed is something dangerous against Celta Vigo, a team that will certainly wait and play with counterattacks to try to prevent Real Madrid from scoring early.
Mourinho has probably thought about facing this game as if it was just another Liga one. First of all, the team needs to win. And if they do so preventing Celta from scoring two goals, the less Real Madrid will get will be extra time. It's a very doable task after Cristiano Ronado's goal in Balaídos, but how the team performs defensively will be very important.
Higuaín admitted that he felt tired after just playing 20 minutes against Real Sociedad, so he's not probably an option to start yet. But considering Callejón's early substitution last Sunday to give Casillas some room when Adán was sent off, the Spanish winger might get a chance to start this game. That way, Ángel Di María will be on the bench ready to turn things around if needed.
One way or another, Real Madrid needs to control the game and let things develop with patience. If the team defends well enough, Cristiano Ronaldo and his teammates shouldn't have a problem scoring a couple of goals on their way to the next round of the Copa del Rey.