Lee Byrne is rated as '50-50' for the Ospreys crunch Heineken Cup quarter-final tie with Munster.
The British Lions candidate sustained an ankle injury in Wales' Six Nations defeat to Grand-Slam champions Ireland last month.
The Wales ace has undergone treatment in an Oxygen tent this week in a bid to be fit for Sunday's clash against the reigning champions.
And although Byrne has been named in an Ospreys 24-man squad he faces a race against time to be fit for the clash at Thomond Park.
Ospreys coach Sean Holley said: "Lee has started training and we will give him every chance. There are encouraging signs and he has made really good progress but he is still touch and go, and 50-50 for the game.
"But with medical science anything can happen. Richard Hibbard was supposed to be out for months but came back within half the expected time."
The Ospreys will make a decision on Byrne's fitness tomorrow (Friday) before flying to Ireland on Saturday morning. If Byrne, 28, fails to recover then Ireland Grand-Slam winner Tommy Bowe or Jonny Vaughton could fill the fullback berth.
But Holley insists the star-studded Welsh region have adequate cover. He said: "We have plenty of options Tommy, Jonny and Jonathon Spratt have all played at fullback.
"Even James Hook has played at fullback at international level with Wales and that shows the strength in depth we have got."
As part of the build-up to next month's Cardiff 2014 Heineken Cup and Amlin Challenge Cup finals weekend, ERC has joined forces with the WRU, Cardiff Blues and adidas to develop an annual grassroots community rugby programme targeting young players.
The Ulster Rugby full back, Jared Payne (No 15), has been suspended for two (2) weeks following an independent Disciplinary Hearing in Dublin arising from the Heineken Cup quarter-final match against Saracens at Ravenhill Stadium on Saturday, 5 April.