The Liverpool Echo reports Alexander-Arnold was sarcastically waving to the home fans who were hurling abuse at him.
And Robertson also enjoyed his ceremonial walk around the pitch as he blew kisses at the United faithful.
Earlier, with his side 4-0 down after just 45 minutes, Cristiano Ronaldo showed his frustration in first half injury time as he aggressively kicked out at midfielder Curtis Jones while the ball was underneath him.
This provoked an instant reaction from the Liverpool players, namely Robertson who sprinted over to Ronaldo and pointed in his face, furious at the 36-year-old's actions.
Robertson was even more motivated following the incident, as was clearly shown when Mo Salah completed his hat-trick to make it 5-0.
The Scot ran over to celebrate with the Egyptian before sending a message of merciless encouragement to those around him.
He could be heard saying: “Keep going, keep going! Be ruthless!"
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