Simon Grayson said that the 29-year old former Leicester City captain will need plenty of time to heal.
"From the start, the assumption with Paddy was that his injury would take months to heal rather than weeks," Grayson told the Yorkshire Evening Post.
"I'm sure he's desperate to get back just as we'd want to have him fit and available but we won't force his recovery and I still don't want to put a timescale on his comeback.
"He'll take as long as he needs to. There's no possibility of him being fit for the start of the season so there's absolutely no reason to rush him.
"I'd much rather he took his time and came back in perfect shape. It'll be a big boost when he does."