Arsenal manager Arsene Wenger admits his team have some defensive frailties to fix ahead of Tuesday's Champions League clash at Schalke 04 as the Germans eye a place in the knock-out stages.
A win over Arsenal at Gelsenkirchen's Veltins Arena will see group leaders Schalke qualify for the knockout rounds with two groups games left with Arsenal currently second.
Goals from Klaas-Jan Huntelaar and Ibrahim Afellay at the Emirates Stadium a fortnight ago sealed a 2-0 win as the German club became the first foreign visitors to defeat Arsenal at home since 2003, ending a 45-game unbeaten streak.
Schalke coach Huub Stevens has said Arsenal remain the favourites to win Group B, but after the Gunners' 2-1 defeat at Manchester United on Saturday in the Premier League, Wenger conceded his team is misfiring.
"At the moment we are not running on all cylinders and are defensively more vulnerable than early in the season," said the Gunners' boss.
Wales midfielder Aaron Ramsey will miss Tuesday's game with a groin strain.
The Welshman was substituted during the second half of the defeat in Manchester and will be replaced by Johan Djourou for the trip to Germany.
Wenger has made one further change with Damian Martinez, who is ineligible for the Champions League, has been replaced by James Shea.
Schalke's Greek defender Kyriakos Papadopoulos is facing a race to be fit after sustaining a cut over his eye in Saturday's shock Bundesliga defeat at Hoffenheim.
The 20-year-old was unable to head any balls during training on Sunday.
Stevens reacted with irritation when asked who would play in goal against Arsenal after Lars Unnerstall, 22, failed to impress in the 3-2 defeat at Hoffenheim, with ex-Germany shot-stopper Timo Hildebrand, 33, on the bench.
"We have three goalkeepers, one will be in goal. Don't always ask the same question. I will not answer it," fumed the Dutchman in Monday's press conference.
But captain Benedikt Hoewedes has said his side must take their chance to at least guarantee second place in the group with a second win over the Gunners.
"The defeat at Hoffenheim exposed a few cracks, but we want to defend our good position in the Champions League," he said.