The Canadian winger injured his knee during his country's 18-9 loss to Romania in Bucharest last weekend and is suspected to have sustained significant ligament damage.
Hassler has scored four tries in just seven league appearances for Ospreys this season, which has helped them to the top of the GUINNESS PRO12 table.
They suffered a first defeat of the campaign to Ulster in Belfast last time out, going down 25-16, but return to Ireland looking to make amends when they face Leinster at the RDS on Saturday.
However, they will square off against the reigning PRO12 champions without their prolific try-scorer as the 23-year-old will be sidelined until at least the start of 2015.
"It will be an extended period of time that we'll miss Hass for," Tandy told the South Wales Evening Post.
"The physios are seeing the surgeons and looking through scans, so we'll have more of an update over the next few days.
"It's a knee injury. I'd say we are definitely talking about him being out into the new year. It will probably be a couple of months into the new year, potentially.
"It could be two months, five months - we just need to get the reports back and see the extent of the injury and then work out time-frames."
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