" Theo Walcott has left Arsenal's training camp in Germany to undergo a scan on a recurring ankle problem.
The England international, 22, sat out training on Monday before the decision was made to send him home.
Arsenal hope the problem will be resolved before they start their Premier League campaign against Newcastle on 13 August.
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We need some more investigation because sometimes he's completely free of it and sometimes it affects him
Arsene Wenger
Manager Arsene Wenger told the club's official website the injury required "investigation".
"Theo went home early and was due to have an MRI [scan]," he said.
"It is an ankle injury that comes and goes.
"We need some more investigation because sometimes he's completely free of it and sometimes it affects him.
"But we had to send him back because he couldn't practise any more."