Arsenal head coach Unai Emery has been handed a major blow ahead of his side's Europa League quarter-final first leg with Napoli after Granit Xhaka failed to recover his fitness in time to feature.
Koscielny and Xhaka had missed both of Arsenal's games following the international break, the former suffering a bruised foot whilst the latter was battling a groin injury. Both had trained with the first-team but only the club captain recovered in time.
Xhaka has not been included in the squad with his midfield place instead taken by the returning Lucas Torreira, who has been banned for Arsenal's last three Premier League matches.
Laurent Koscielny issues injury update ahead of Arsenal's crucial Europa League tie with Napoli
Matteo Guendouzi and Mohamed Elneny had both started at Everton on Sunday, in a 1-0 defeat, but neither have retained their places with Torreira being partnered by Aaron Ramsey, who completed 45 minutes at Goodison Park.
Still the return of Koscielny is a crucial addition to a backline that struggled to deal with the threat of Everton at the weekend.
"I am available again after missing the last two matches," the club captain said.
"I had a problem on my foot and my thigh and I didn't want to risk aggravating the injuries by playing against Newcastle and Everton."
The Alexandre Lacazette and Pierre-Emerick Aubameyang problem Unai Emery's happy to have for now
Pierre-Emerick Aubameyang has been confined to the substitutes bench for the last two of those games as he battled an illness. A course of antibiotics were required for the striker, who has been confined to appearances off the bench against Newcastle and Everton, but now returns to lead the line alongside Alexandre Lacazette.
Meanwhile Lorenzo Insigne has shaken off his injury in time to start for Napoli.
Arsenal XI: Cech; Sokratis, Koscielny, Monreal; Maitland-Niles, Ramsey, Torreira, Kolasinac; Ozil; Aubameyang, Lacazette
Subs: Leno, Mustafi, Elneny, Guendouzi, Iwobi, Mkhitaryan, Suarez
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