The Croatian is unable to feature against his former club after failing to shake off a hamstring issue suffered a couple of weeks ago. He was missing for the games against Brighton last week and West Ham at the weekend, though should have recovered in time to play in the second leg in Spain.
Apart from that, Thomas Tuchel has a full squad at his disposal for the huge Champions League encounter.
In goal, Edouard Mendy returned to the starting XI in the weekend win versus the Hammers and will likely retain his spot after Kepa Arrizabalaga started a couple of games.
Cesar Azpilicueta will start on the right of the back three, having featured slightly further forward at wing-back against Chelsea. Thiago Silva could drop out as the Brazilian doesn't tend to start two matches in four days for the Blues.
That could mean a return for Reece James.
Ben Chilwell's defensive prowess is favoured over that of Marcos Alonso so it could be the case that the Englishman keeps his spot at left wing-back, even though Tuchel does like to rotate that position.
In the absence of Kovacic; N'Golo Kante and Jorgnho should partner each other in the centre of midfield.
Tuchel has plenty of options for his front three, with Mason Mount a certain starter.
Timo Werner grabbed the only goal of the game against West Ham and will probably keep his spot, though Christian Pulisic will likely drop to the bench with either Kai Havertz or Hakim Ziyech coming in as his replacement.