André Santos has apologised to his Arsenal team-mates and the club's fans for taking the shirt of Manchester United's Robin van Persie as he left the Old Trafford pitch at half-time on Saturday but he has received a dressing-down from Arsène Wenger and is expected to be dropped for the Champions League tie here against Schalke on Tuesday night.

Wenger, the Arsenal manager, will not base his selection on the furore that Santos sparked by claiming Van Persie's shirt, rather his concern at the Brazilian's form at left-back; Santos endured a torrid time in the 2-1 defeat against United. Wenger said that the shirt saga had nothing to do with the result and he is aware of the practice among South American players of swapping shirts at half-time.

He was embarrassed, though, by the context of Santos seeking the souvenir from Van Persie, his former Arsenal teammate, whose goal had made the half-time scoreline 1-0, and he has taken steps to ensure that it will not happen again.

"I spoke [to Santos] about that but I do not want to make too big a story of it," Wenger said. "It was not the right thing to do at all but that is not an explanation for our bad performance. There is a difference of culture. I can understand that an English guy doesn't understand it and I'm French and I don't understand it, either.

"He is completely in a different world when he does that. He understands today what it means and he will never do it again. But it's interesting because Santos is a player who cares, he is serious and he wants to do well. He gives an image of himself [on Saturday] which is not him."

Santos was quoted by the Sun: "I'd like to apologise to supporters who felt aggrieved by it. Did I think it would cause such a commotion? Of course I didn't, otherwise I would have told Robin to exchange the shirts away from public view."

Wenger is set to move Thomas Vermaelen to left-back, in place of Santos, and recall Laurent Koscielny in central defence. Kieran Gibbs is injured. "We think deeply about everything before we make those sort of decisions," Wenger said, when asked whether he would make changes against Schalke. "We will have to sort that problem out anyway."

Wenger has a sensitive selection decision over Theo Walcott, whom he has not started in either the Premier or Champions League since the player's contract talks stalled in late August. Aaron Ramsey damaged his groin against United and he has joined Alex Oxlade-Chamberlain, Gervinho, Tomas Rosicky and Abou Diaby on the midfield injury list.

Wenger, though, reported that Walcott had not trained on Monday because of a stomach virus and he would need to assess him. Walcott travelled to Germany as part of an 18-man squad but if he cannot play, Wenger would have only six fit substitutes. "The decision is nothing linked with his contract at all," Wenger said. "It's a digestive problem."