Alexander-Arnold rushed away from the goal to celebrate with Diaz after having a hand in its creation.
He did this while booting the ball, which came close to smashing De Gea directly in the face.
The Manchester United goalkeeper was forced to take evasive action and quickly reacted by grabbing the England right-back by the collar.
Alexander-Arnold then confronted the Spaniard and shoved him in the face as the two exchanged insults in a tense encounter.
Liverpool captain Jordan Henderson was quick to intervene and drag Alexander-Arnold away, while referee Martin Atkinson also intervened to defuse the situation - though he did not show Trent any punishment for his actions.
Mo Salah added a second goal for Liverpool on 22 minutes, while tensions remained calm at the time.
On the seventh minute, the two sets of rival supporters came together to pay an emotional tribute to Cristiano Ronaldo on the field.
Following the heartbreaking death of his young boy, the entire stadium erupted in applause for the Manchester United player.
WATCH VIDEO AFTER THE CUT...
Jubilee Ohwodiasa 1 y
Bro gotta chill