Henson has undergone surgery on a dislocated bone in his right wrist, a problem suffered during the World Cup warm-up victory over England on Saturday.
He will be sidelined for six to eight weeks, and by the time he regains full fitness it is possible Wales could be on their way home from New Zealand.
Wales coach Warren Gatland names a 30-man World Cup squad next Monday. If, as seems certain, he is not selected, then Henson could only feature in the tournament by replacing an injured player.
Centre Henson has never played in a World Cup. He was overlooked by then Wales boss Steve Hansen for the 2003 tournament, and then failed to recover in time from injuries four years later. When the 2015 competition comes around, he will be 33.
It is the latest setback for Wales after wing Morgan Stoddart broke his leg against England at Twickenham nine days ago, while last season's Test skipper - hooker Matthew Rees - is battling to overcome a neck problem.
Wales, meanwhile, say fly-half Rhys Priestland, who left the pitch at half-time against England nursing a thigh injury, will be re-assessed today.
But both full-back Lee Byrne and wing Leigh Halfpenny have been declared fully fit and in contention for selection against Argentina in the final World Cup preparation game next Saturday.
And Stephen Jones, Ryan Jones and Gethin Jenkins, who all have calf muscle problems, will join Priestland in reporting for injury assessment, when further details about their progress will be established.
"Gavin underwent surgery at the Vale Hospital to correct the dislocation to a bone in his right wrist," said Prav Mathema, the Welsh Rugby Union's national medical performance manager.
"He has been given a return to play time of six to eight weeks which, while ruling him out of selection for the initial World Cup squad, could see him available to the coaches while we are still in the pool stages of the tournament.
"With that fact in mind, Gavin will remain under the care of the Wales national squad medical staff in order to ensure that he returns to fitness at the earliest opportunity in case the coaches need to call him out to New Zealand for any reason."
Wales kick off their World Cup campaign against world champions South Africa in Wellington on September 11, followed by appointments with Samoa, Namibia and Fiji.
Henson, who is currently without a club after spells at Saracens and Toulon last season, impressed during his 30 minutes on the pitch as Wales overcame England 19-9 in Cardiff.
But the injury is another career dip for Henson, who spent almost two years out of the sport as he came to terms with repeated fitness blows.
During that time, he embarked on a television career, taking part in programmes such as the BBC's Strictly Come Dancing, and there were question marks over whether he had lost his rugby appetite.
But he participated in two Wales training camps in Poland last month, and looked to have done enough to secure his World Cup place until injury struck again.
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