Rovers have revealed the former England international, 33, suffered serious respiratory complications following a back operation last week.
He was kept in hospital over Easter after developing a pulmonary embolism, but has now returned home to recover.
It means Rovers have been dealt a double blow as fellow keeper Jake Kean, 22, sustained torn cartilage in his left knee in Monday's 3-0 defeat by Cardiff.
The player needs an operation and will miss the rest of the season.
Polish keeper Grzegorz Sandomierski, 23, came on to replace Kean for his debut and is now set to be thrust into the npower Championship relegation battle.
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