Sergio Ramos was replaced in the 70th minute of Real Madrid's 3-1 win over Napoli on Wednesday after taking a knock to the right knee.
Karim Benzema, Toni Kroos and Casemiro netted for Madrid after Lorenzo Insigne's extraordinary goal for Napoli less than 10 minutes into the tie at Santiago Bernabeu Stadium.
The second leg will be in Italy on Tuesday, March 7.
Zinedine Zidane had urged his side to keep a clean sheet to give them the best possible chance of going through, but those hopes were dashed after only eight minutes when Insigne embarrassed goalkeeper Keylor Navas with a stunning first-time effort from distance.
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Officials say numerous passengers were trapped under the crushed bus, and rescue personnel used a crane to pry the vehicle open. No proper cause of the accident is investigated yet but it is said that a speedy driving may b a cause of an accident.
But Insigne had other ideas and he put Napoli in front when he raced onto a flawless threaded through-ball by Marek Hamsik before spotting goalkeeper Keylor Navas way off his line and stroking a low bouncing effort into the net from fully 35 yards.
Cristiano Ronaldo has failed to score in his last 523 minutes of Champions League action, his longest drought in the competition at Real Madrid.
Real Madrid had a chance to level in the 12th minute when Ronaldo again found Benzema inside the Napoli box, but the striker's header was away from goal as the visitors survived a risky moment. There didn't look too much danger when a loose ball fell to defensive midfielder Casemiro but the Brazilian took the ball first time on the volley to send it arcing over Pepe Reina.
Hamsik could have made Real pay five minutes later but he fired just wide with a good chance from an Insigne cross.