The Arsenal full-back Andre Santos has been reprimanded for rushing to grab Robin van Persie's shirt at half-time in Saturday's game against Manchester United and must now wait to see whether a dreadful run of form costs him his place in tonight's Champions League tie away to the group leaders Schalke.

"I don't like it and it will not happen again," the manager Arsene Wenger said last night. "I believe that was not the right thing to do at all, and it's not welcome but that is not an explanation for our bad performance. He's a very genuine player who wants to do well."

It would have been undiplomatic of any player to make such a high priority of collecting a shirt at the interval when Arsenal were a goal down and being outclassed, but many supporters were particularly infuriated that it was Santos, who has been so bad recently that a Sunday newspaper awarded him one mark out of ten for Saturday's effort; The Independent was more generous, with a two.

In the 2-0 home defeat by Schalke a fortnight ago he was also caught out of position repeatedly as the German side played diagonal passes behind him for Jefferson Farfan to run onto. That rare home defeat by foreign opposition – a first in more than 40 games – put Schalke top and left Arsenal vulnerable to Olympiakos, who are three points behind but should win tonight at home to the bottom club Montpellier.

Wenger's side cannot therefore afford another defeat or the sort of laxity that had him admitting of the Old Trafford game: "We were very poor defensively." Unfortunately Kieran Gibbs is not yet sufficiently fit to return at left-back, so either Santos must play or Thomas Vermaelen, also poor on Saturday, will have to move to left-back, which he dislikes, with Laurent Koscielny brought back in – even though the Frenchman looked equally hapless as Reading scored five against him in the Capital One Cup last week.

The recent defensive woes are in contrast to the start of the season, when a back-line of Carl Jenkinson, Per Mertesacker, Vermaelen and Gibbs conceded one goal in four games. Bizarrely, Arsenal still have the best record in the Premier League, which of the evidence of the past few games is no more than a statistical quirk.

One expected change further forward was that the injured Aaron Ramsey would at last be replaced by Theo Walcott, who has first to overcome a stomach upset. Six years ago a young Walcott was among those sitting in the Veltnis Arena here and watching as England went out of the World Cup on a penalty shoot-out. Sven Goran Eriksson had controversially taken him along just for the ride and declined to use him even as one of the shock troops, which, was just about understandable.

Wenger's recent preference for Ramsey is inexplicable and has led to speculation that Walcott is being punished for refusing to sign a new contract at the salary on offer. "It has nothing to do with it," the manager said last night. "He has been sick and injured and is only just coming back, so he has not been available." Walcott has, however, regularly livened up Arsenal's labouring attack when brought on as a substitute and against Reading he scored a hat-trick to become Arsenal's leading scorer with seven; only to be immediately left out on Saturday.

Schalke lost 3-2 at Hoffenheim at the weekend despite dominating the game. Their goalkeeper Lars Unnerstall, was criticised for two of the goals and is under pressure for his place from the 33 year-old Timo Hildebrand. But the team is not expected to differ much from the side who dominated at the Emirates.

Probable teams

Schalke: UNNERSTALL, UCHIDA, HOWEDES, PAPADOPOULOS, FUCHS, FARFAN, JONES, HOLTBY, NEUSTADTER, AFELLAY, HUNTELAAR

Arsenal: MANNONE, VERMAELEN, KOSCIELNY, MERTESACKER, SAGNA, PODOLSKI, ARTETA, CAZORLA, WILSHERE, WALCOTT, GIROUD

Kick-off 7.45pm, Veltins-Arena. TV Sky Sports 2

Referee N Rizzoli (It)

Odds: Schalke 4-5 Draw 12-5 Arsenal 3-1

Group B

Results so far

Montpellier 1-2 Arsenal
Olympiakos 1-2 Schalke
Arsenal 3-1 Olympiakos
Schalke 2-2 Montpellier
Arsenal 0-2 Schalke
Montpellier 1-2 Olympiakos.

Remaining fixtures

6 Nov: Olympiakos v Montpellier
Schalke v Arsenal
21 Nov: Arsenal v Montpellier
Schalke v Olympiakos
4 Dec: Montpellier v Schalke
Olympiakos v Arsenal.