The 19-year-old impressed in the 2015/16 Guinness PRO12 season, and was expected to be a heavy part of the upcoming campaign, but anterior cruciate ligament damage is now expected to keep him out until at least April.
Ospreys finished eighth last term, but Watkin provided a bright spark to their campaign, with head coach Steve Tandy giving the centre 13 appearances in which he crossed the try line twice.
Watkin picked up the injury in training, with medical performance manager Chris Towers confirming fears that the Wales Under-20 international has ruptured his ACL.
The teenager represented his country in the Under-20 Six Nations Championship which came to its conclusion in March, helping Wales claim their first-ever Grand Slam at that level and earning a contract with his region just a month later.
By then Watkin had represented the Ospreys on six occasions as an under-18, making his debut for the senior side against Munster in September last year.
His first try came against the same opposition, coincidentally his first win with the region in February, celebrating his first senior contract in April with a try just two days later against Treviso.
Watkin will undergo surgery in the coming weeks, while Ospreys begin their Guinness PRO12 campaign against Zebre on September 2.
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