The assist came from an unlikely source, Dejan Lovren.
The defender headed the ball back across the box following a corner and Mane was on hand to stick the ball past Keylor Navas.
Marcelo will not want to watch the replays, he was slow to get out of defence, losing his man and playing the Senegal forward onside.
It was the perfect tonic for Liverpool, who looked on the brink after Mo Salah's injury and the opening goal.