The Canadian back damaged his knee ligament in a clash with Romania, but initially it was hoped that he would be able to return for the first half of next year.
However the Welsh region now expect they will be without Hassler, who had enjoyed a productive start to the GUINNESS PRO12 campaign with four tries in seven games, until next season.
The news comes as Ospreys look to recover from two losses on the spin after an unbeaten start - but Rees believes his side can maintain their menace from the wings without the 23-year-old.
"We aren't expecting to see Jeff again this season," he said.
"It's a blow for him and for us because of what he offers, but injuries happen and you just have to try to adapt and minimise the disruption.
"Eli Walker is coming back to fitness and our other wings have been playing well, too.
"But I feel for Jeff because things had been going so well for him."
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Guinness PRO12 Ospreys News
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