Tests have confirmed the Blues back will need surgery to repair the damage and subsequently will sit out the rest of the World Cup, with Newport Gwent Dragons centre Tyler Morgan called up as a replacement.
And Allen, who made 14 appearances in the GUINNESS PRO12 last season, says he's still coming to terms with the setback.
"I had a scan the morning after which pretty much confirmed what everyone expected, that I'd torn my hamstring and there was also a grade three tendon tear as well," he said.
"I went up to London to see a specialist and his advice was that I have an operation. I'm back off to London on Tuesday to have the op.
"The injury is a massive disappointment but these things happen."
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Guinness PRO12 Blues News
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