Keane suffered cartilage damage in the goalless draw with Notts County last midweek and went under the knife on Wednesday.
North End boss Graham Westley is upbeat over the full-back's recovery prospects after revealing the injury was not quite as bad as first feared.
"He's been under the knife and everything was successful," Westley told BBC Radio Lancashire.
"The news was better than we thought it might be, in that he's probably going to be out for three months rather than the four or five that we suspected."
The Lilywhites also lost Keane's fellow defender Scott Laird to a broken leg against the Magpies.