The contrasting fortunes for British clubs in Europe continues on matchday one in the Europa League.
Koscielny scored via an overhead kick to equalise for the Gunners against Southampton.
Santi Cazorla's stoppage-time penalty gave the Gunners the three points.
French boss believes Saints deserved at least a point at the Emirates Stadium.
Arsenal beat Southampton 2-1 in the league.
The Gunners required a last-minute penalty to win at the Emirates.
Santi Cazorla held his nerve to score from the spot in the last minute.
Arsenal host Southampton in the Premier League.
The Gunners signed the duo shortly before the transfer window closed.
Claude Puel has confirmed summer signing Jeremy Pied will be sidelined for between six and nine months.
But the Portugal international refuses to dis failed move to Old Trafford.
Gunners midfielder keen for first-team football opportunities away from the Emirates.
The Morocco international signs a five-year deal at Saint Mary's.
Group A will also see the experienced striker face another former club in Eredivisie outfit Feyenoord.
Manchester United placed in Group A alongside Fenerbahce, Feyenoord and Zorya Luhansk.
Jose Fonte led the Portugal side to win the Euros.
The centre-back is a reported transfer target of Manchester United.
The Southampton captain has been linked with a move to Old Trafford.
The Southampton midfielder was replaced by Jordy Clasie after just 11 minutes of 2-0 loss at Old Trafford.
Zlatan Ibrahimovic notches a brace in Jose Mourinho's first home match in charge.
Paul Pogba likely to feature for the first time since rejoining Manchester United.
The £89m signing was banned for the season opener against Bournemouth.