Sydney Swans midfielder Ryan O’Keefe’s preseason has been dealt a blow, with the 31-year old suffering a knee injury at training on Tuesday.

O’Keefe has some bruising on his left knee and is likely to miss the next 3-6 weeks.

Coach John Longmire said it was a disappointing setback for O’Keefe.

“Ryan has had a great preseason. He hasn’t missed a beat before this setback, so he is really fit,” he said. 

“Bruising can be difficult to assess so while the diagnosis is 3-6 weeks on the sidelines, we’re hopeful he could be up and running in two weeks if the knee responds well to some rest." 

After undergoing off-season groin surgery, O’Keefe returned to his best in 2011, playing all 24 games. He was a standout with four goals in the elimination final win against St Kilda and finished fourth in the Club Champion award.

While O’Keefe will be missing in action, Longmire has declared Shane Mumford a certain starter for Saturday night’s clash with North Melbourne at Bruce Purser Reserve.

Josh Kennedy, Andrejs Everitt, Gary Rohan are also in the mix to make their returns at Rouse Hill.