Without being able to tell if the mass is solid or not, it's hard to determine exactly what the problem might be. However, nine times out of ten it's usually an abscess the buck has acquired by busting through brush or skirting under cattle fencing. I've always been under the assumption that this doesn't effect the meat for human consumption.
There are literally dozens of afflictions game animals can have which can't be field diagnosed......but when in doubt, process the animal and take a portion down to DFG to get it tested (don't know if they charge, but a black bear I took a long time ago was free. It of course had trichinosis)