Manager Arsene Wenger says there will be 'further investigation' carried out on the ankle injury ahead of the new Premier League campaign, which gets under way in just over two weeks.
"Theo went home early and was due to have an MRI on Tuesday," Wenger told Arsenal's official website.
"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."