Theo Walcott is set to miss Arsenal's next three fixtures after a setback in his recovery from abdominal surgery.
The winger recently underwent a minor operation that ruled him out of England's final two FIFA World Cup qualifiers, but manager Arsene Wenger has revealed that the problem flared up again in training.
"He had a setback. He had to stop, to jog again, I don't think we will see him against Dortmund, nor Crystal Palace nor Chelsea," he said.
"He is at least two weeks away now."
With Andros Townsend making a big impression in England's victories over Montenegro and Poland, Walcott will be keen to make his return before the the friendlies against Chile and Germany in November.
Meanwhile, Wenger refused to write off Manchester United's Premier League challenge after David Moyes' side's disappointing start to the season.
"Manchester United are not out of the title race. We are only two points in front of all the others," he added.
"Are Man United out of the title race? No, but of course it starts to be a little bit of a gap.
"If you have a choice nobody wants to be 10 points behind. If you have the choice, you want to be in the race at the top.
"But you cannot rule Man United out. They have big players, big experience and they are a big club."
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