Virgil van Dijk chested a frustrated Cristiano Ronaldo after he was involved in an incident with Curtis Jones and the five-time Ballon d'Or winner didn't quite fancy it.
At the end of a shambolic first-half where United were torn to shreds, Ronaldo lost his head big time and kicked out at substitute Jones.
The ball was there but there was clear aggression from Ronaldo and many felt he should have been dismissed.
Liverpool players were straight in to confront United's No.7, including Van Dijk. The Netherlands captain and all of his 6ft 3 frame, went straight over to Ronaldo and chested him out the way.
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Ronaldo, quite visibly, was a little taken aback and Liverpool fans enjoyed their defender stepping up.
Van Dijk was pushed out the way by Bruno Fernandes, who then got into it with Ibrahima Konate after he arrived on the scene.
United, who were four goals down at half-time in the Premier League for the first time in their history, were utterly humiliated by their rivals at Old Trafford.
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Mo Salah scored a clinical hat-trick, with Diogo Jota and Naby Keita also on the scoresheet in the rout.
Paul Pogba, who had been brought on as a substitute for Mason Greenwood half-time, was sent off for a shocking challenge that forced Keita off on a stretcher on the hour mark.
Featured Image Credit: Image: Sky SportsTopics: Manchester United, Liverpool, Virgil Van Dijk, Cristiano Ronaldo