The South African-born wing has scored 14 tries in 42 games for the region but is facing surgery on a cartilage problem in his right knee.
The loss of Dirksen is a big blow for the Ospreys with the region having lost a pair of inspirational wingers at the end of last season when Lions duo Shane Williams and Tommy Bowe departed for pastures new.
Having represented USA at age-group level he will qualify for Wales in 2014.
His rehabilitation period is expected to be six months and while that will hit the RaboDirect PRO12 outfit hard physio Chris Towers admitted the decision was a pragmatic one.
"We've spoken to several specialists and the view is it is in Hanno's best interests to have the surgery now," said Towers.
"We've taken a long term view on what is best for Hanno's career.
"At this stage we are probably looking at a post-op rehab process of around five to six months, it's not something we can rush."
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