During Preston's Championship clash against Derby, the Republic of Ireland international suffered a serious knee injury.
The 26-year-old, who appeared in 40 league games last season, will face a long period ofrehabilitation.
Preston boss Alex Neil has absolutely no doubt that the defender will return stronger after his bad injury. "As I described him after the game on Tuesday night, Greg is a 'warrior' - as illustrated by the fact he wanted to carry on despite having done the damage that now requires an operation", he said.
"I have no doubt he will be back fitter and stronger before very long and our medical team will work closely with him to get him back with us", Neil added.