Ancelotti will have to make a decision to replace Xabi Alonso. He has stated that Illarramendi would be the obvious replacement, but the former Real Sociedad midfielder has not been confident lately. Therefore, Real Madrid could try to give Khedira some more minutes against Espanyol and see if he's able to complete the whole match.
Nobody expected him to play 60 minutes against Vigo, and with two more weeks to get in shape, Khedira will surely try to help his teammates in Lisbon. Some argued that his presence would change the team's formation to a 4-4-2 that isn't that great when your team is going to have the ball. But Khedira's experience and defensive work could be useful for Real Madrid. Furthermore, he's a good aerial player, and that's one of Atlético de Madrid's biggest threats. All things considered, to choose Khedira would not be as risky as to put Illarramendi or Casemiro on the pitch.
He would also give Modric and Isco or Di María more freedom to dictate the tempo of the game. Khedira's physical presence could match Koke, Gabi and Raúl García's intensity. If Ancelotti feels Khedira is ready, the German player will surely be the one starting the Champions League Final.