London - Arsenal will be aiming to dispel concerns about its defensive strength when it hosts Olympiakos in the Champions League on Wednesday.
Arsenal's unbeaten start to the season ended on Saturday when Chelsea capitalised on weak and uncertain defending at set pieces to win 2-1 in the Premier League.
It's an area where Arsenal appeared to have improved this campaign but manager Arsene Wenger was highly critical, saying the defending was "unacceptable" and lacked authority.
Germany centre back Per Mertesacker could be recalled in place of Laurent Koscielny, having been dropped for the Chelsea game despite his good early-season form, as Arsenal and Olympiakos meet in the Champions League group stage for the fifth match in four seasons.
"I think there is some psychological damage," Wenger said of the Chelsea loss. "What is important is the personality you show in a big game, and that is where in some areas we were not at the level."
After opening with a 2-1 victory at Montpellier on match day one, another win two weeks on will put Arsenal in a solid position to secure a top-two finish and qualify for the knockout stage for the 13th season in a row.
German team Schalke is also in a strong position after beating Olympiakos 2-1 away in its opener and now faces Montpellier at home.
If Arsenal's defense is to be breached on Wednesday, it is likely Rafik Djebbour will be the player to do it. The Algeria striker has scored a league-high six goals in Olympiakos' five straight opening wins in Greece's top division, which have lifted the team to first place in the standings.
However, with key striker Kevin Mirallas having been sold to Everton in August and experienced Swedish defender Olof Mellberg also recently leaving the club, Olympiakos may not have the quality to seriously challenge Arsenal and Schalke in the group.
Roy Carroll, the former Manchester United and West Ham goalkeeper whose career has been rejuvenated by leaving the Premier League to play in Greece, could face a busy night.
"My last United appearance was against Arsenal, in the 2005 FA Cup final, when they won on penalties," the 35-year-old Northern Irishman said. "It's frightening to think they've won nothing since. Trophies are expected at a club of their stature.
"But we are top of the Greek league with five straight wins and playing well. There is a lot riding on it for us, me especially. I have waited a long time to play at one of English football's top venues again, and I intend making the most of it."
Arsenal will be without France midfielder Abou Diaby, who is out for a minimum of three weeks - according to Wenger - after hurting his thigh against Chelsea.
Schalke heads into the match against Montpellier following a disappointing 2-2 draw with promoted Fortuna Duesseldorf on Friday, when the 2011 Champions League semi-finalists squandered a two-goal lead.
"We have to forget about the Duesseldorf game as quickly as possible and concentrate on the Champions League," midfielder Jermaine Jones told the Bundesliga website. "We have to learn from the game because such mistakes are ruthlessly exploited in the Champions League and you get punished even more."
Montpellier's start to the defense of its French league title isn't going smoothly, with the team in the bottom half of the standings even after a 2-0 win at Nancy on Saturday.