Liverpool boss Jurgen Klopp believes his team had issues with its rhythm against Atalanta.
The Reds are still in the fox seat in their group, with nine points from four games.
Atalanta and Ajax now have seven a piece, with the Italian side getting a 2-0 win over the Reds at Anfield on Wednesday.
Klopp believes heavy rotation may have hampered his team in this game.
Asked why the team struggled, he told the club's website: "Rhythm, we didn't have rhythm tonight.
"We made five changes and that obviously gives you a few problems with rhythm, but we usually get through a game.
"Atalanta were good – Atalanta fought, Atalanta defended well, that's why we didn't have a shot on target.
"My defence was completely fine apart from two situations, that's how it is.
"Atalanta did do well in these moments and we have to admit [that].
"It is a deserved defeat, I have said that a couple of times. It doesn't sound nice but it's the truth, so congratulations to Atalanta."