Arsenal midfielder Aaron Ramsey is out and Theo Walcott is doubtful for tonight's Champions League Group B clash at German side Schalke 04.

Ramsey was substituted during the second half of Arsenal's 2-1 defeat at Manchester United on Saturday and he was replaced by Johan Djourou in the squad for the trip to Gelsenkirchen.

The Welshman's injury should have opened the door for Theo Walcott to regain a starting spot after the England winger was consigned to the bench in recent weeks following an impasse on signing a new contract.

But Walcott is doubtful through illness.

"He has been sick on Sunday and in practice on Monday, and we will have to see how he is Tuesday morning," Wenger said.

Wenger is hoping the Schalke match will mark a turnaround in his team's season after recent bad results.

"At the moment we are not running on all cylinders and are defensively more vulnerable than early in the season," said Wenger.

Schalke top Group B with seven points and are one ahead of Arsenal after they won 2-0 in London last month.