Cattermole has had an operation on his knee, while Scotland international Fletcher picked up an ankle ligament injury on international duty. It is not known if Fletcher's ankle problem will require surgery.
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"Cattermole underwent an operation on his knee earlier today and Fletcher sustained ankle ligament damage whilst on international duty with Scotland.
"It is not known at this stage if Fletcher will require an operation, with the injury requiring further assessment in the coming weeks.
"Both players will play no further part in the Black Cats 2012-13 Barclays Premier League season."
The Black Cats currently sit 15th in the Premier League, just four points clear of the relegation zone and without a win in seven games.
Next up for Martin O'Neill's side is a home game against Manchester United, followed by a trip to face Chelsea.