The 36-year-old has suffered a recurrence of the medial ligament issue that ended his season at Sunderland in November last year.
The former Republic of Ireland international, who joined Hibs on a one-year deal earlier this summer, is staring at the possibility of being out until at least October.
“It’s a disappointing one,” manager Lee Johnson said as he revealed the news about McGeady at his pre-match media briefing ahead of Saturday’s cinch Premiership opener away to St Johnstone.
“He’s going to be out for a sustained period of time after aggravating his previous medial ligament injury. It’s hard to put a timescale on it but it’s going to be minimum six weeks, although it probably averages about 10 weeks.
“I’m disappointed for Aiden first and foremost. He’s worked extremely hard to get back to a level of fitness that is capable of performing at Premiership level and unfortunately this has happened.
“It’s not ideal but it’s why you have a squad. Somebody else has got to fill that void on a leadership level and a performance level. We just do our best to recover him as quickly as possible.”
Johnson also explained long-serving skipper Paul Hanlon’s new ‘club captain’ role with David Marshall appointed team captain.
“Paul Hanlon is a key part of the club moving forward, not just short term but medium term and potentially long term. I think it’s a great role for him with higher responsibility. He’ll be a bit of a daddy of the club, if you like, and it’s a role he can dine off for a long time if he does it well.
“The fact Ryan Porteous is going to start concentrating on that leadership element as well is important as it helps him channel that aggression into the controlled aggression we want. Also his will to win can be a positive for us.”