The Gunners were beaten at home on the first day of the Premier League season for the second year in succession and had a number of players unavailable as Wenger handed extended breaks to those involved at the European Championships.
Theo Walcott missed a penalty before making amends 69 seconds later to give Arsenal the lead before Philippe Coutinho levelled on the stroke of half-time with a superb free-kick.
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Adam Lallana put the visitors ahead before Coutinho and debutant Sadio Mane extended their lead - with Alex Oxlade-Chamberlain and Calum Chambers grabbing goals for Arsenal to make the scoreline more respectable.
Wenger was forced to play Chambers alongside 20-year-old summer signing Rob Holding in the heart of his defence with Per Mertesacker and Gabriel both injured.
And he also opted against involving Laurent Koscielny, Olivier Giroud and Mesut Ozil in his squad after their exertions at Euro 2016 - while Aaron Ramsey, who reached the semi-finals with Wales, was forced off with a hamstring injury in the second half.
"Physically we are not ready," Wenger conceded.
"You are in a catch 22 situation with the Euros - do you give the players a rest and start the season without many of your players? Or do they play in this kind of game and get injured, like Ramsey today?
"We had many players that played very late in the Euros - Ramsey, Koscielny, Ozil, Giroud - they went to the semi-finals (at least). We had four players who went very late."

Arsene Wenger on the touchline for Arsenal

Image credit: Reuters

Wenger also lost winger Alex Iwobi to injury and, with Granit Xhaka the only other signing along with Holding, he admits he does not know who he will have fit for the trip to face champions Leicester at the King Power Stadium next weekend.
"We will try, but when they have not enough preparation they get injured," he said of his rested players.
"You could see that we dropped physically today. It was a game of contrast between two halves but I felt as well that mentally we kept going and we have been absolutely remarkable on that front.
"We have to be ready next week because we go to Leicester. But we'll see first what we can do with the players who are preparing at the moment and how many players are injured today - can they recover for next week? I don't know.
"But we are not stupid. We prepared well the players we have in but I think you have to consider we have been a bit unlucky as well.

Per Mertesacker in action for Arsenal

Image credit: AFP

"We have lost Mertesacker and Gabriel in preparation, and Koscielny is not fit. You have to sometimes accept that you cannot control absolutely everything, even if you try to be as intelligent as possible."
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