Flop: Dries Mertens

Carlo Ancelotti's decision to play the Belgian striker in place of Arkadiusz Milik backfired as his lack of game time showed and rarely ever threatened the hosts.
His link-up play was sloppy and lacked the firepower to make an impact in the final third. In fact, Mertens didn't even make a single effort all night, let alone direct one on target. His 26 touches on the ball was also the lowest among the 22 players.
Mertens has been a wellspring of goals for Napoli over the years, but this season he's been relegated to the bench by an in-form Milik. And this cap on opportunities seems to have taken a wrecking ball to his confidence. He even though scored thrice in the previous four games, never appeared to add to that tally.
Playing Mertens alongside Lorenzo Insigne was meant to stretch open Arsenal defense with their pace, but the visitors crossed the ball out wide instead. So when Milik was introduced in place of Mertens in the 66th minute, Napoli looked more threatening and created a couple of decent chances.