The Spanish champions trail Barcelona 3-2 going into the second leg at the Santiago Bernabeu Stadium after losing the first game in Camp Nou last week.
Real spent a €35 million (Dh161m) on Modric, but while they have yet to find their form going forward, their significant problems have been at the back as a defence, missing Pepe, has struggled, conceding five goals in two losses.
Modric has already trained with the team and is ready to step in wherever needed by the coach Jose Mourinho, who is unlikely to return out-of-favour midfielder Kaka to the line-up for tonight's game.
"I'm ready, but coach will decide if he use me or not," Modric said. "It's going be tough for me to earn a place but that's the challenge, the biggest challenge of my career. Expectations are as high as possible and I'm ready for that."
Mourinho will be hoping Pepe has recovered from a concussion after Raul Arbiol failed to convince he merits a place in the backline.
Real's performance at Getafe on Sunday was ineffective and Mourinho may hope Modric's arrival provides some kind of a spur for his side with Barcelona in great form to begin the season.
The Catalans have won all three of their matches and have already opened up a five-point gap on their great rival.
The Copa del Rey champions would have been in firm control of the Super Cup if not for a late error by their goalkeeper Victor Valdes that allowed Real to pull back an extra away goal after they had deservedly moved 3-1 ahead.
Tito Valdes, the Barcelona coach, defended his goalkeeper and said: "I'm surprised that anyone at this level can doubt Valdes.
"The club has four Champions League trophies, and he was decisive in three of those."
The defender Carles Puyol could be available despite a fractured right cheekbone after returning to training yesterday, while Thiago Alcantara also trained for the first time since May, and the midfielder Alex Song could make his debut after arriving from Arsenal last week.
Gerard Pique said he and his teammates would not get carried away if they do convert their strong position going into the second leg into their first silverware of the season and their fourth successive triumph in the competition.
"The season couldn't have started any better for us," the Barcelona defender, said.
"[But] the Super Cup is separate to the league. It's important to win but I don't think the title will affect the face of the league."