Wales lock Luke Charteris will miss the Six Nations after undergoing an operation on a wrist injury.
There is a fear that Alun Wyn Jones, who with Charteris formed Wales' first-choice lock partnership at the World Cup, could also miss the tournament.
Ospreys captain Jones has had surgery on a toe he dislocated in training.
"I had an operation on Friday and, according to the surgeon, it went OK. They said I was looking at four months [out]," Charteris said.
The versatile Ryan Jones can also play in the second row as well as the Osprey's preferred back-row spot.
Alun Wyn Jones injured his toe in training while Charteris picked up his injury, a similar wrist problem to the one Wales centre Jamie Roberts suffered in July 2010, in Wales' bronze medal play-off defeat by Australia at the World Cup in New Zealand.
Charteris, 28, will also miss the remainder of the Dragons's Amlin Challenge Cup campaign.
Jones, 26, is now expected to miss the rest of the Ospreys's Heineken Cup fixtures - and Gatland fears he will miss Wales' Six Nations opener against Ireland in Dublin on 5 February.