The 27 year-old suffered a horror ankle injury in a Championship match against Millwall at the Weston Homes Stadium last December.
Butler is currently with most of the Posh first-team squad at the training camp in Portugal, but he's six-to-eight weeks from a return to action according to manager Grant McCann. The League One season starts on July 30.
Young centre-back Emmanuel Fernandez is not in Portugal after suffering a weaked hamstring in training, while long-term injury victim Ollie Norburn is continuing his recovery from knee ligament surgery in the UK.
McCann said: “Dan is a litle bit behind in his rehab. I don’t really want to put a timescale on it, but he’s probably six-to-eight weeks away from returning, and maybe a little bit more.
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"We are pushing him a bit this week with running and gym work. He’s a great character and he’s working hard.
"Manu just felt a tweak in his hamstring and so we felt it best to leave him behind.”
Butler’s absence could well lead to Joe Tomlinson starting as a left-sided defender in the team that travels to Cheltenham for the League One opener.