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.
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."
Walcott made 44 appearances for club and country last season, scoring 13 goals, but he suffered injuries which hampered his progress. A hamstring problem in February meant he missed six weeks of the season, including the Carling Cup Final.
In September he injured his ankle playing for England against Switzerland, again ruling him out of action for six weeks.
Meanwhile, Wenger also confirmed midfielder Conor Henderson, 19, is set to miss at least six months of the season.
The Republic of Ireland Under-21 international was taken off on a stretcher in the closing stages of Saturday's 2-1 win over Cologne and his injury has now been confirmed.
Wenger said: "Conor has a knee injury and it looks as if it is a cruciate [ligament].
"That means unfortunately that he will be out for six months at the very least."