The Irishman has endured a torrid time with injuries since moving to Highfield Road from Middlesbrough last summer.
He has suffered groin, back and hip injuries during his Sky Blues tenure, restricting him to just 11 appearances, and none since last year.
O'Neill was making a comeback towards the end of the past campaign but his injury hell showed no signs of abating when he suffered a horrific setback in training.
The 26-year-old dislocated his ankle and broke his leg in a collision with team-mate Lee Mills and the club have confirmed he will be ruled out of fledgling manager Gary McAllister's plans until December.
It is merely the latest setback for O'Neill and his colleague John Eustace admits it is wretched luck for the Irishman.
"It does not look as if Keith will be available until Christmas time after suffering a very nasty injury in training," Eustace told the club's official website.
"He had a lot of bad luck with injuries and just when he was on his way back he suffered another setback.
"All the lads wish him well and want to see him on the road to recovery so he is back midway through next season."
