Liverpool face the threat of crashing out at the Champions League group stage after a shock 2-0 defeat to Red Star Belgrade in the Serbian capital.
Milan Pavkov scored twice in seven first-half minutes to hand Red Star their first victory in the European showpiece in 26 years.
The defeat exposes last year's runners-up to the threat of an early elimination in Group C, with Napoli coming from behind to draw 1-1 at home to Paris Saint-Germain. Liverpool and Napoli are locked on six points from four games, with PSG one behind and Red Star another point adrift. Liverpool travel to Paris for their next game.
"The boys are very disappointed, I'm very disappointed and we have to do better," Reds manager Jurgen Klopp said.
"[Red Star] had too many set-pieces and with one of them they scored. Then more or less straight away they score the second one and then the atmosphere was completely different, you could feel it."
Barcelona, meanwhile, secured their place in the knockout stage from Group B with a 1-1 draw at Inter Milan,
Mario Icardi's 87th-minute strike for Inter cancelled out Brazilian Malcom's opener for the visitors.