Shane Williams is Wales' record try-scorer with 48 scores in 68 caps
The Ospreys fear Shane Williams will miss their final four Heineken Cup pool fixtures after suffering a hamstring tear in Wales' defeat to Australia.
The Lions wing limped off in the fifth minute of Wales' 33-12 defeat to the Wallabies and scans have revealed a hamstring tear and a six-week layoff.
Williams joins Wales team-mates Mike Phillips and Lee Byrne on the Ospreys injury list for their European games.
But captain Ryan Jones is to recover from a back injury to play Viadana.
The Wales skipper suffered a back niggle in training but despite intensive treatment, Jones missed Saturday's Millennium Stadium humiliation to the Aussies.
Jones could sit out the Ospreys' Magners League clash with Munster in Swansea on Saturday but is set to be fit for the Ospreys Heineken Cup double-header with the Italians.
"Ryan will link back up with the Ospreys this week and should be taking some part in training and conditioning but will obviously be managed by his region," said Wales team physio Mark Davies.
The Ospreys will play Viadana in a Heineken Cup Pool Three clash in Reggio Emilia on 12 December before the Liberty Stadium return on 19 December.
And the Welsh region's final European pool games against Clermont Auvergne and Leicester Tigers are in mid to late January.
So Williams, who is also the Ospreys' back-up scrum-half, faces a race against time to be fit for those final Euro fixtures - and means the region must go into their Heineken Cup games with Viadana with just one scrum-half.
Injuries to Phillips, Jamie Nutbrown and Tom Issacs means that new South African scrum-half Ricky Januarie is the only registered fit number nine in the Ospreys' Heineken Cup squad.
Ospreys director of coaching Scott Johnson admitted: "It is always disappointing when you lose a player of Shane's calibre through injury, but this news is a particular blow for us given the current situation we have here regarding our scrum halves.
"To have lost all three of our named nines for the tournament is an incredible situation, which resulted in us having to bring in emergency cover in the form of Ricky Januarie last week.
"With tournament regulations meaning that we are only able to make one squad change during the pool stages, as well as playing in his regular position on the wing, Shane was going to have to provide our only cover at scrum half, so his absence is a particular blow."
Williams, who does not require surgery to aid his recovery, is not just Wales' record try-scorer but is the Ospreys' top Heineken Cup try-scorer.
His score - and 11th try for the region in Europe - in their Heineken Cup draw at Leicester Tigers earlier this season is his only try for his region this term.
Ospreys physio Chris Towers confirmed: "Shane picked up a left hamstring injury during Saturday's game, something which was pretty evident to anybody watching the game at the time.
"The WRU organised a scan for him yesterday to look at the extent of the injury. Based on the findings of the scan, we could be looking at him being unavailable for a period of four to six weeks while he recovers from the injury."
Williams' injury is a minor blow for Wales coach Warren Gatland also as the Ospreys star would have had limited game-time before their Six Nations opener against England at Twickenham on 6 February.
Jones' namesake Adam Jones could also join his skipper on the field against Viadana as the Lions and Wales prop is expected to make his return from shoulder surgery in the Ospreys' crucial European game.
The 28-year-old was injured playing for the Lions in their 28-25 second Test defeat to world champions South Africa in June.
Lions full-back Byrne is expected to return in the new year after suffering ligament damage in his right foot in the Ospreys' 32-32 Heineken Cup draw with Leicester.