Wales' Ryan Jones suffers fresh injury blow
Wales' Ryan Jones is pictured on February 24, 2007 at the Stade de France in Saint-Denis, north of Paris
The Ospreys forward has made just one appearance this term, facing Italy's Treviso in the Celtic League on the opening night of the season and lasting just 48 minutes before having to go off injured.
Jones had been hoping to play against Irish province Ulster on Friday but will now miss that match as well.
"The problem Ryan has is that he trained last week and pulled up with a calf strain, to go on top of what he's already been dealing with," Ospreys' forwards coach Chris Gibbes told the South Wales Evening Post.
"How serious is this latest bump? It's enough for him not to be able to play. We are just going to need to assess where he's at as we go through this week.
"We feel for him. He's a huge part of our team and to have a guy like that involved in the squad is massive.
"It's unfortunate for Ryan, but other guys will have their opportunities."