Having seen Liverpool beat Atalanta 5-0 in Bergamo on 4 November (AEDT), Jurgen Klopp selected a blend of fringe players and first-team regulars in his starting line-up and paid the price as the Premier League champion failed to muster a shot on goal until the 44th minute.
It got worse for Liverpool after half-time as Gian Piero Gasperini's well-organised Atalanta side scored in quick succession through Josip Ilicic and Robin Gosens on Wednesday.
On a night when victory would have qualified Liverpool from Group D with a 100 per cent record, the standings are instead tightly balanced with Klopp's team just two points clear of Ajax and Atalanta, who both have seven.
But it will be the subdued nature of Liverpool's performance that most concerns Reds manager Klopp.
Mohamed Salah, back in the team after a negative coronavirus test, gave the ball away sloppily in the middle of Liverpool's half after three minutes, allowing Ilicic to curl a shot wide from 20 yards.
Robin Gosens burst into the box moments later and forced a strong one-handed save from Alisson with a thunderous shot.
A Salah strike that drifted tamely over the crossbar one minute before half-time was Liverpool's only attempt of the first period – its lowest tally of shots in a first half since January 2017 against Chelsea.
Atalanta continued to press early in the second half, with Papu Gomez testing Alisson with a low drive, and the breakthrough arrived just before the hour mark.
The Italian visitor was given space and time around the Liverpool penalty area before Gomez curled an in-swinging cross to the edge of the six-yard box, where Ilicic slid in to poke a low shot past Alisson into the corner of the net.
The second came four minutes later and was an even better move, Hans Hateboer heading a looping Gomez delivery back across the face of goal where Gosens opened up his body and volleyed past Alisson.
Klopp sent on Andy Robertson, Roberto Firmino and Diogo Jota to try and turn the game, but Atalanta held firm to give their hopes of reaching the knockout stage a huge boost.