Andy Robertson urged Liverpool to "wake up quickly" after a dire first-half performance resulted in a 4-1 humbling at Napoli in their Champions League opener on Wednesday.
Liverpool have failed to live up to early expectations in the Premier League, winning just twice in six games, and their poor start to the season continued in Naples.
Piotr Zielinski converted from the penalty spot after just five minutes, before Alisson denied Victor Osimhen's spot-kick – but that only denied the inevitable for the out-of-sorts Reds.
Andre-Frank Zambo Anguissa and Giovanni Simeone, the latter on his Champions League debut, added further goals before half-time, with Zielinski adding a fourth after the interval.
It was just the fourth time Liverpool have found themselves three goals down at half-time in the competition, with Luis Diaz's second-half strike making little difference for Jurgen Klopp's side.
The thrashing marked the Reds' joint-heaviest defeat in the Champions League, and Robertson offered an honest appraisal after the game.
"We were miles too open. You can't come to a place like this and not be compact. They were by miles the better team," the Scotland international told BT Sport.
"Too many times they had spaces to run at us and cause us problems. You come away in the Champions League and you can't be wide open like that. We have to get back to basics and be compact.
"We deserve this result, even though we created chances. The way we were wide open they found so many spaces, that's not like us. It felt like they had an extra man.
"You have to be ready to fight. We weren't close enough to our mate when they went to press the ball, people weren't backing each other up soon enough.
"They hit the post within the first minute then they get a penalty, and another one and you're 3-0 down. We had two good chances to make it a bit different – but you can't dwell on that.
"We have to wake up quickly because we can't perform like that."
Liverpool will look to respond at home to Wolves in the Premier League on Saturday before hosting Ajax, who smashed Rangers 4-0 in Group A's other game on matchday one.