Christian Pulisic's performance against Liverpool on Sunday was easily good enough to earn a Man of the Match award, but there was big one problem: he wasn't old enough.
Pulisic, still just 19-years-old, has not yet reached the legal drinking age in the United States.
Liverpool had a €13 million bid rejected for Pulisic back in August 2016 and, although Klopp believes the 19-year-old is still unavailable for a transfer, he admits he remains a fan of the US global.
"We probably could have taken more chances but we were definitely the better side and in the second half they got back into it and they managed to finish off their chances".
Tickets for the El Clasico last year between Barcelona and Real Madrid sold for over US$10,000 each, although demand this year in the early stages of the competition hasn't been quite so high.
The former Chelsea man should have had an assist before Dortmund drew level, as he neatly played in Sheyi Ojo for a one-on-one opportunity but the latter's shot was diverted onto the crossbar by goalkeeper Marwin Hitz.
Liverpool manager Jürgen Klopp has provided an update on the injury picked up by defender Joel Matip in pre-season. I think it was the third goal, he couldn't do it anymore. "I like Christian, I've known him since he was a kid and he's still not really old".
Dortmund came into the clash on the back of a 1-0 win over Manchester City on Friday evening, with Mario Gotze netting the victor.
Better was to follow for the American, a target for Sunday's opponents, as he put the Germans into the lead with a minute to play.
His misery was compounded with the signing of highly rated Roma goalkeeper Alisson, who will go into the new Premier League season as Liverpool's goalkeeper.