The Frenchman returned to the side in the 1-0 victory over Qarabag for the first time since rupturing his Achilles in May's defeat to Atletico Madrid.
Koscielny, 33, played 72 minutes and enjoyed a relatively quiet game in central defence before being replaced by Nacho Monreal.
And he is ready to kick-start his season under Unai Emery and help extend Arsenal's unbeaten stretch that has reached 22 games in all competitions.
He said: "It was my last day of rehab, so now I start a new chapter in my career.
"For me it is important to enjoy the game with my team-mates, and it didn't matter about the result, it was just about being on the pitch with my friends.
"It is difficult being out injured. It was the first big injury of my career and I had to take a long time out, but you learn a lot about yourself and I am very happy.
"I am focused mentally, and I can smile and enjoy the rest of the season.
"It was very important to keep our unbeaten run going for the confidence and now we start a big challenge ahead of Christmas.
"Today we played with many young players and they showed lots of determination and focus on their jobs."
His return from injury has come just at the right time for boss Emery.
Arsenal are suffering a mini crisis in central defence ahead of their Premier League clash with Southampton on Sunday.
Shkodran Mustafi is sidelined with injury joining Rob Holding and Konstantinos Mavropanos on the treatment table.
Sokratis Papastathopoulos will also miss out with suspension.
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