Arsenal received the Frenchman's scan results on Monday night after the swelling had subsided on the ankle injury he sustained against Manchester City on Saturday.
Goal understands that Debuchy has been diagnosed with a grade three ankle sprain, the most severe type that involves a torn ligament.
Although there will be relief that the France international has not broken a bone, he is expected to be sidelined for between eight to 12 weeks.
It is a huge blow for Arsene Wenger, whose already meagre defensive resources have now been further weakened.
With deputy right-back Calum Chambers struggling with tonsillitis, Wenger is ready to turn to another rookie, Hector Bellerin, for Tuesday's Champions League group opener against 2013 runners-up Borussia Dortmund.
Bellerin, 19, who is yet to make his full Arsenal debut, is part of the travelling party that arrived in Dortmund on Monday evening.
Debuchy is the latest long-term absentee for Arsenal, along with Theo Walcott (knee) and Olivier Giroud, who is out until at least November with a broken foot.
It leaves Laurent Koscielny, Per Mertesacker and Kieran Gibbs as the only fit senior defenders in the Gunners squad.
Nacho Monreal will miss the Dortmund match with a back injury but is expected to be available for the weekend trip to Aston Villa.
Wenger had said earlier on Monday he was unsure of the length of Debuchy's absence as he awaited the player's scan results but admitted his resources are stretched.
Wenger said: "In defence the Debuchy injury puts us a little bit in a situation where we do not want more. Hopefully we will have good news after he has a scan and that he will be back soon.
"We have Chambers who can play in different positions, Monreal can play centre back and we also have Bellerin who I think is now ready to play."
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