According to the club's official website, the experienced stopper has developed a pulmonary embolism, which will rule him out for the next six months.
The statement read: "Paul is set for a lengthy spell on the sidelines after suffering serious respiratory complications following a back operation last week.
"The former England international developed a pulmonary embolism (blood clot in the lung), which will rule him out for the next six months. Robbo has now returned home after being held in hospital over the weekend."
Jake Kean will also miss the rest of the season with a knee injury.