A first-half brace from Mohamed Salah put Jurgen Klopp's men 2-0 up at the Tottenham Hotspur Stadium.
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The first was vintage Salah as he collected Darwin Nunez's pass before firing past Hugo Lloris.
His second was a lovely clipped effort over the Spurs goalkeeper, however, this came after Dier dropped a clanger as his attempted head back to Lloris fell straight into the Egyptian's path.
On Dier's error, Darren Ambrose told talkSPORT: "What is he doing? What's gone through his head?
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"Just head it out, why are you trying to head it back when Mo Salah's running in behind you?
"You're told from a young age if in doubt clear it away from danger."
Former West Ham centre-back Anton Ferdinand added: "Where he is on the pitch, he's quite far from his goalkeeper so trying to head it back he's asking for trouble.
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"You're facing where Mo Salah is coming from so you should see him in your peripheral vision a player like Mo Salah running.
"The best thing to do is head the ball away."