Theo Walcott has been forced to fly home with an ankle injury.
Walcott will now undergo further scans with the new Premier League season just two weeks away.
"Theo went home early and was due to have an MRI on Tuesday," Wenger said.
"It is an old ankle injury from the end of last season. It's one 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."
At least it is not a new injury, but this is not how he wanted to start the new season. Especially with Gervinho ready to play on the right of a front three.