The 26-year-old, who has made over 200 appearances for the Premiership club, has been out of action since the end of September. However, his hopes of a quick return have been scuppered by the news he requires a reconstruction of the anterior cruciate ligament of his left knee.
"It is desperately disappointing for him and for us, but it is a straightforward operation and he should make a full recovery," said Southampton physio Jim Joyce.
Oakley's absence will be a blow to Southampton's chances of securing a European place for a second successive season. The south-coast team are currently ninth in the Premiership with 17 points from 12 games. They were knocked out of the UEFA Cup in the first round by FC Steaua Bucuresti.
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