William Gallas may have a Baker's Cyst.
From Wenger's description, the defender does not appear to have a straightforward calf strain. There is fluid accumulation that is putting pressure on his calf muscles which seems suspiciously like the complication from a Baker's cyst rupturing. These cysts are the possible sequelae to a history of knee ligament and meniscal injuries.
Whatever the cause, It looks like Gallas will be out for the next 4-5 weeks. If that happens then he will not be available for the CL quarterfinals against Barca.
The injury complicates contract talks. Gallas wants a two year deal which is contrary to Wenger's usual offer of a one year contract as a matter of policy to players over 30 years. The only way out for Gallas is to take a hefty wage cut. An injury might be additional leverage but then at this crucial stage why expose Sol Campbell?
The good news is that Cesc and Tomas Rosicky have been declared fit and they will see action in the weekend against the Hammers. Alexander Song is back from his two match suspension. So we have our midfield back.