Liverpool defender Andrew Robertson criticised Dominik Szoboszlai’s mistake in Monday’s FA Cup win over Barnsley, calling it “unacceptable,” while acknowledging that the Reds still have room to improve despite their 11-match unbeaten run.

Szoboszlai was guilty of a careless back-heel inside his own penalty area that allowed Barnsley’s Adam Phillips to score during the third-round tie at Anfield. Liverpool were already 2-0 up at the time and went on to secure a 4-1 victory over the League One side.

Earlier in the match, Szoboszlai had opened the scoring with a superb long-range strike. Liverpool manager Arne Slot described the back-heel as a “weird” decision and said he would speak to the Hungarian midfielder.

Robertson was equally frustrated with the lapse in concentration, which prompted Barnsley manager Conor Hourihane to accuse Szoboszlai of disrespect.

“The frustrating thing is Dom does this unbelievable 60-yard sprint to start with, he’s getting the clap off The Kop, and I don’t know if that maybe went to his head,” Robertson said.

“We can joke about it now, but it’s unacceptable to lose a goal like that. Dom obviously knows that. He’s been magnificent for us this season and was really good again, but he just had a lapse of concentration. We can’t afford too many of those in the six-yard box.”

Liverpool have now gone 11 matches unbeaten, helping ease the crisis that engulfed the Premier League champions earlier this season. But Robertson admitted the Barnsley game highlighted a tendency to allow teams back into matches.

“Going 11 unbeaten is good, but some of the performances still have to be better. Everyone knows that,” the Scotland left-back said.

“We’ve looked a bit more secure defensively, which is pleasing, but now we need to put it all together. People are saying we’re lacking an attacking threat. If you want to win trophies and go further in tournaments, you have to find the perfect balance. That’s what we’re working towards.”