Atletico took charge of the round 16 tie with this first-leg 5-2 win.
Alvarez, named man of the match, said afterwards: "It was a complete move. First (Jan) Oblak's great save, then Antoine (Griezmann)'s magic, and then I received the ball. Their defender gave me space to move forward, and I finished well.
“We knew how to take advantage of our opponents' mistakes to gain a good lead, and now we go into the second leg with a three-goal advantage.
"You can see we're doing things right and improving all the time. The pressure we apply often pays off. When the whole team pushes forward, it creates plenty of chances, and you have to take advantage of opponents' mistakes.
"That's what leads to moments like this."
