(Clarifies 10th para)
* Napoli twice came back only to lose 3-2 to Besiktas
* Aboubakar's double included late headed winner
* Besiktas goals all came from poor Napoli defending
* Insigne missed a penalty for Napoli early in the second half
* Second Napoli penalty converted by substitute Gabbiadini
* Besiktas host Napoli in next round of matches
MILAN, Oct 19 (Reuters) - Vincent Aboubakar scored twice, including an 86th-minute winner, to give Besiktas a shock 3-2 win at Napoli on Wednesday after the hosts had twice come from behind and missed a penalty.
Napoli's defence was at fault for all three Besiktas goals as the Serie A side slumped to a third successive defeat in all competitions and lost an 18-match unbeaten home record in Europe.
Napoli would have become the first team in Champions League history to qualify from the group stage with three matches to spare if they had won and the other match, between Dynamo Kiev and Benfica, had been drawn
Instead, they remained on six points and are now just one ahead of Benfica, who won 2-0 at Dynamo Kiev, and Besiktas with half the group still to play.
Napoli were unbeaten in their first eight matches of the season but it has all gone wrong following a knee injury to striker Arkadiusz Milik, himself brought in to replace Gonzalo Higuain following the Argentine's move to Juventus.
Having unsuccessfully tried Manolo Gabbiadini as Milik's replacement in the 3-1 defeat by AS Roma on Saturday, Napoli fielded Dries Mertens as a false number nine.
"It's tough to lose like this," Mertens told reporters. "We've given everything and we just didn't have the luck. It's not because Milik is absent, we're still creating plenty of chances."
The Belgian had two early chances but Besiktas went ahead with their first opportunity in the 12th minute when the unmarked Adriano turned in Ricardo Quaresma's low cross at the far post.
The lively Mertens then wasted another chance, heading wide from close range, but finally hit the target on the half-hour, stabbing the ball home Jose Callejon's pass after the Spaniard was sent clear by Marek Hamsik's superb lofted pass.
Besiktas went back in front eight minutes later, however, when Jorginho tried to play the ball back from the halfway line but gave it straight to Aboubakar who burst clear and fired his shot past Pepe Reina.
The drama continued after halftime as Besiktas goalkeeper Fabricio saved Insigne's weak spot kick in the 50th minute, the Spaniard atoning after giving away the penalty in the first place.
Napoli were given another chance in the 69th minute after Caner Erkin hauled down Mertens and this time Gabbiadini, who had replaced Insigne, sent Fabricio the wrong way.
Gabbiadini had a goal disallowed for offside as Napoli pressed forward in search of a winner but Besiktas stole the points when Cameroon forward Aboubakar stole in front of his markers to head in Ricardo Quaresma's free kick from close range. (Writing by Brian Homewood; Editing by Ian Chadband)