The 27-year-old was given an extended break after playing a part in Spain's Confederations Cup campaign, which has seen the defender miss the Gunners' pre-season tour of Japan.
But during that time off, Monreal has damaged his back and, although Arsene Wenger does not know what caused the injury, he now expects the former Malaga man to miss the start of the Premier League season.
"Honestly I don't really know," Wenger said of Monreal's injury. "[Laurent Koscielny] got an injection and it looks like he won't be able to start the season.
"That is why, of course, we lost one left-back and that is the position where we are a bit exposed at the moment. [Thomas] Vermaelen can play there but he is injured, Monreal can play there, but he is injured, so we only have Kieran, at the moment, who can play there."
Arsenal's defence could be very undermanned for the start of the 2013-14 Premier League season, with goalkeeper Wojciech Szczesny also struggling with a lower back problem.
Vermaelen is expected to miss the next three months with his own back troubles, while Koscielny has injured his ankle.