Dani Carvajal started in the first leg but has since suffered a hamstring injury, and with Lucas Vazquez also still sidelined, Alvaro Odriozola is the only recognised right-back that Zidane can call upon.
Nacho Fernandez could play in that role, but the Spaniard may be forced to fill in at centre-back after Raphael Varane was taken off injured against Osasuna, and it remains to be seen how serious his apparent leg issue is.
Sergio Ramos could be available for the second leg despite missing out at the weekend, although given his fitness concerns throughout the campaign, Zidane may be reluctant to risk his captain from the first whistle, meaning that Nacho and Eder Militao could pair up in a back four.
Marcelo should be available for the tie despite being called up for polling duty, although he may be cutting it close as he flies in on the morning of the match, and there is also optimism over Ferland Mendy's fitness as he aims to shake off a calf issue.
Toni Kroos and Luka Modric are both set for recalls to the middle of the park after being rested against Osasuna, but Federico Valverde reportedly returned an inconclusive coronavirus test recently, so he is expected to miss out.
Eden Hazard started on Saturday but may drop out against his former club after an underwhelming showing, while Karim Benzema will hope to overtake Raul as the Champions League's fourth highest goalscorer of all time following his 71st goal of the tournament in the first leg.
Real Madrid possible starting lineup:
Courtois; Odriozola, Militao, Nacho, Mendy; Casemiro, Kroos, Modric; Asensio, Benzema, Vinicius Jr
> Click here to see how Chelsea could line up for this clash