The England international missed a training session earlier in the week but manager Arsene Wenger hopes to sort the recurring problem before the start of the new season.
"Theo went home early and was due to have an MRI," he told the club's official website.
"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 any more."
The Gunners begin their Premier League campaign on Saturday, August 13 away at Newcastle United.