I don't have any problem with people speculating that Mayo might not be back next year. I think he will be, but it's definitely a real possibility that he's done with the Pats. Cutting him wouldn't make the top 10 for most unexpected personnel moves. My guess would be that they try to do something like what they did with Vince this offseason, where they extend his contract and lower his cap hits for the next couple of years in the process. The big difference, though, is that unlike with Wilfork this past offseason I think it would be pretty reasonable for the Pats' to keep Mayo even without a restructure. He's really good, and you're not going to find a better linebacker for the money you'd save by cutting him.
I think it depends at least in part on what the injury actually is. If it's a torn ACL, it's much more likely that they just honor the existing contract on the relatively comfortable assumption that he'll make a fully recovery. If it's a ruptured patellar tendon, a 100% recovery becomes much less of a formality, and some real risk re: not making a full recovery enters the equation.