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.
Alexis Sanchez equalised for the visitors, scoring on the rebound after Manuel Neuer saved his 30th-minute penalty, but that seems of little outcome now before the second leg in London on March 7.
Barca were smashed at PSG on Tuesday night and Real were stung when Lorenzo Insigne fired the Italians ahead after 12 minutes.
But Real Madrid battled back to make it 1-1 at the half when Karim Benzema headed home Dani Carvajal's cross 10 minutes after the opening goal.
Real will feel good about this result, but that away goal means Napoli still has life.
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The remainder of the half was spent mainly in Napoli territory as Real continued to press, with Cristiano Ronaldo blasting over after being set free by Luka Modric and Benzema clipping the outside of the post when through on goal.
This goes to Kroos, who got his goal and his passing in the middle really improved as the game went on, allowing Real to quickly transition to attack.
Napoli started the second period on the front foot, but it was Real Madrid that scored the game's third goal in the 49th minute when Ronaldo dribbled towards the away side's box before picking out Kroos, who simply passed the ball into the bottom corner of the net.
Insigne stunned Real Madrid early on with a heads-up play.
Both managers made changes as the match entered its final 15 minutes, and it was Real Madrid that were searching for the game's next goal as Benzema and Ronaldo started to find each other in unsafe positions once again.