Monreal was given an extended break after playing a part in Spain's Confederations Cup campaign, which meant the 27-year-old defender was not taken on Arsenal's tour of Japan.
But during that time off, Monreal has injured his back and although Arsenal manager Arsene Wenger does not know what caused the injury, he believes the former Malaga man will miss the start of the English Premier League season.
Monreal's injury means Wenger may only have one left-back option for Arsenal's league opener against Aston Villa - Kieran Gibbs.
"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 (lower back) also injured.
Vermaelen is expected to miss the next three months with his own back troubles, while Koscielny has injured his ankle.
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