The Reds pulled off one of the most famous comebacks in Champions League history, winning 4-0 at Anfield having trailed 3-0 from the first leg at Camp Nou.
To pile on the misery for Messi, Robertson was involved in a minor disagreement with the 33-year-old and bizarrely ended up ruffling his hair - something the Scottish full-back now regrets.
“We both ended up on the ground and I ruffled his hair a wee bit," he told That Peter Crouch Podcast.
"He wasn't too happy, but it's something I wouldn't do again. It's just disrespectful to the best player that's ever played the game.
"I do regret it, but it's something that everyone relates to me now. I wasn't thinking clearly."