Arsenal manager Arsene Wenger has revealed forward Theo Walcott has left the club's training camp in Germany to undergo a scan on a recurring ankle problem.
England international Walcott, 22, sat out training on Monday before the decision was made to send him home and try to fix the problem before Arsenal start their Barclays Premier League campaign against Newcastle next month.
Wenger, who will start the new season hoping to guide his side to a first piece of silverware in six years, told the club's official website the injury required 'investigation'.
"Theo went home early and was due to have an MRI," 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 practice anymore."
Meanwhile, Wenger also confirmed young midfielder Conor Henderson is set to miss at least six months of the season.
The Republic of Ireland Under-21 international was stretchered off 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. That means unfortunately that he will be out for six months at the very least."
Arsenal's pre-season comes to a head this weekend when they play Boca Juniors and New York Red Bulls in the fifth Emirates Cup, and Wenger is looking forward to clashing with the 23-time Argentinian champions.
"The South Americans will cycle the ball at a slow pace and suddenly try to kill you, so it's always a good experience - and they have all the tricks of street football too," he added.
"They can slow the game down, they always have good technique and intelligent behaviour at the back. It's always difficult to score goals against them. Boca Juniors is a great name of world football, so it will be interesting."