On another day, Mane might have had a hat trick.

Bear with me on this.
First half, VVD had a near post header, lacking in pace, straight at the keeper. Had he glanced it across the face of the goal, Mane was unmarked at the back post.
First half, Romeu's studded attempt, straight at the keeper. If he had got less of a touch, the ball could have squirted across goal to an unmarked Mane, at the back post.
Second half, as already mentioned on the match day thread, had Martina hit a hard, low cross along the 6 yard box, Mane was unmarked at the back post.
Sometimes you just need a little bit of luck, such as an accidental pass to set you up with a two yard tap in. He showed that he still has the instinct to be in the right place.