Super-sub Dries Mertens came off the bench to create one and score the second as Napoli ensured top spot in Group B with a 2-1 win over Benfica in the Champions League.

Having replaced the misfiring Manolo Gabbiadini on 57 minutes the Belgium international created Jose Callejon's opener with a deft flick before sealing the three points late on.

Napoli's winning margin could have been even wider had Gabbiadini - who has now failed to score in seven games in all competitions - and Callejon done better in the first half.

Callejon was not to be denied on the hour, though, as he finally showed some composure to score his first goal of this season's competition.

Substitute Mertens added the decisive second with 11 minutes to play with a low drive that was perfectly placed to beat the dive of Ederson.

Raul Jimenez grabbed a consolation for the hosts but Napoli were able to hold on to ensure they go into the last-16 draw as group winners, while Benfica also progress after 10-man Besiktas were hammered 6-0 by Dynamo Kiev

Napoli looked the more likely early on as they looked to attack at pace but it took until the 21st minute for them to create the first real opening.

Lorenzo Insigne fed Marek Hamsik 20 yards from goal, the Slovakian’s low shot was parried by Ederson into the path of Gabbiadini who headed home. However, the goal was ruled out as the striker was rightly flagged offside.

That let-off seemed to spark the hosts into life and a minute later Goncalo Guedes should have done better with a snapshot after Napoli twice failed to clear their lines.

It was a fleeting threat from Benfica, though, as Napoli finished the half stronger with Callejon and Gabbiadini both denied by Ederson before the break.

News of Besiktas' troubles in Kiev appeared to relax the hosts after the restart and Callejon should have punished them having been found by Insigne, but he snatched at his volley and skewed wide.

Just before the hour Napoli coach Maurizio Sarri opted to introduce Mertens in place of Gabbiadini and three minutes later the Belgian created the opening goal, Callejon racing onto his flick and lifting the ball over Ederson.

Spurred on by the goal Napoli continued to threaten and Insigne nearly executed a sublime chip from the edge of the penalty area in the 66th minute.

Pizzi then wasted a chance to test Pepe Reina with a long-range free-kick and Napoli made Benfica pay as Mertens wrapped things up.

The 29-year-old drifted past the challenge of Luisao before rifling a low shot past Ederson, the ball clipping the post on its way in.

Benfica halved the deficit late on as Jimenez pounced on a mistake by Raul Albiol and fired beyond Reina, but it was not enough to stop Napoli becoming the first Italian side to win in the Champions League on Portuguese soil since 1992-93.