The Belgian converted a quick corner by Trent Alexander-Arnold to complete a 4-0 win over the Spanish giants.
Matip told the club's website: "I was slowly on the way to the box, so in centre-back style I was slowly going from back to front! I thought maybe in 20 seconds there would be a corner coming in or something like this; I came forward and had a nice view of the whole situation.
"It was crazy. I couldn't believe it, I didn't know what had happened because he just put the ball in. I thought, 'What is this?' Then I saw Divock there and the ball in the net afterwards…
"I was a little bit confused, to be honest. But after a few seconds, everybody was celebrating so I just joined in."