The goal seemed to inspire Koumas further as he burst into the area between two defenders and tried to set up Glatzel, who was blocked out.
The Reds held their advantage comfortably until just before the break, when Athletic Club stirred. A headed finish was ruled out for offside after a cross from the left, but the visitors soon had their equaliser.
Again a delivery from the left brought the threat, headed back across goal and into the danger area. Fabian Mrozek made a great save to deny an initial shot, but former Academy attacker Elijah Gift was on hand to fire home, declining to celebrate against the club he left last summer.
Lewtas brought on Kieran Morrison at the break and the Northern Ireland youth international almost had an immediate impact with a shot on the turn that was too close to Gastesi.
But it was the opposition who produced the telling contributions of the half.
Mrozek did well to turn away a low shot from Bari but could do nothing on 56 minutes as Athletic Club took the lead, Asensio turning well in the box and shooting low into the net.
They might have stretched that advantage 10 minutes later. A lovely one-two at the edge of the area left striker Inigo Lopez just a few yards out from goal, but he somehow put his effort wide.
Glatzel had a speculative strike go close and Calum Scanlon fired over from a decent position as Liverpool sought an equaliser.
It almost arrived with 12 minutes left from another fine Davidson cross, Koumas setting up Morrison – whose clever attempt was just too high.
But that was as close as the hosts came and deep into injury time, Athletic Club grabbed a third. A corner was played to the edge of the box, where a shot was deflected several times before landing at the feet of substitute Asier Hierro, who made no mistake from close range.
TEAM
Liverpool U21s: Mrozek, Miles, Scanlon, Pinnington, Nallo, Pilling (K. Kelly, 85), Kone-Doherty (Figueroa, 85), Davidson, Cannonier (Morrison, 46), Hill (Glatzel, 8), Koumas.
Unused subs: O. Kelly, Spearing.
NEXT UP
The U21s will be swiftly back in action as they travel to Brighton & Hove Albion on Saturday afternoon. The Premier League 2 contest is a 4pm GMT kick-off at the Amex Elite Performance Centre.
Sophie Anderson, a UK-based writer, is your guide to the latest trends, viral sensations, and internet phenomena. With a finger on the pulse of digital culture, she explores what’s trending across social media and pop culture, keeping readers in the know about the latest online sensations.
