Despite defeat, Liverpool finish top of the league phase and can now concentrate on domestic matters - where they are top of the Premier League, in the semi-finals of the EFL Cup and the FA Cup fourth round - before their next European game in March.
With Liverpool and Barcelona finishing first and second respectively, the two clubs cannot meet until the final.
Mohamed Salah, captain Virgil van Dijk and Trent Alexander-Arnold were among those given the night off as Jayden Danns became the youngest player ever to make his first start for Liverpool in a European Cup/Champions League game at the age of 19 years and 13 days.
Danns was one of four players aged 21 or younger to start while Trey Nyoni, 17, and Nallo, 18, came on as substitutes.
Nallo will not forget his senior debut in a hurry.
He lasted three minutes and 46 seconds, with the young defender managing just two passes before he was sent off.
"It is cruel but he has never played first team football before and to make your debut in the Champions League is probably the hardest," said Slot.
"A career isn't always positive, it also has negatives and he has to make sure he plays in this competition again."
Despite the result and Nallo's red card, Liverpool march on and are sure to wheel out their main players when they next play in the competition in the first week of March.