Koumas assisted Bradley Barcola's opening goal before scoring Liverpool's sixth in the 72nd minute in what was an incredible game for the 21 year old who has stepped up under Andoni Iraola.
After both Alexander Isak and Hugo Ekitike picked up injuries, Koumas has been trusted by Iraola to start as a striker for the Reds in what has been a genius move so far from the Spaniard.
The Welsh youngster became the second player in Liverpool's history aged 21 or under to score and assist in the same Champions League match.
Club icon Steven Gerrard first achieved the feat away when he was also 21 years old against Dynamo Kyiv in October 2001.
Gerrard, who is clearly a fan of the academy star, praised Koumas before the youngster's Champions League goal, following Liverpool's 3-0 defeat to Manchester City last week.
“He'd done himself proud, he'd done the academy proud. He didn't look like an academy player out there, he played like a man.
“I'm proud of him. I really am. I'm delighted that he took his chance. What he showed me as a Liverpool fan yesterday is he can be part of this group moving forward, because he will improve."
Iraola also praised Koumas who will be seeking to start once more against Brentford this weekend as he continues his excellent run.
"He has to enjoy it because he deserves it. I think he scored two disallowed goals which is unlucky for him but he kept pressing, making the runs in behind and doing his job.
"I think it’s fair that he score his goal and can have his moment of celebration with the supporters."