I wouldn't expect any one guy to be a direct replacement, like you might've been looking for in detroit, etc.
they've basically had a logjam of bodies in the front 7, and pretty much each week somebody would lose snaps as a result --- week 1 ninkovich came off the field, then I think collins was hurt, so he was the guy, now it's been hightower missing time
really, their 'sub' defense is their base, and their 'base' defense is out there some small amount of time, so we're talking about a lot of nickel, maybe 5-2 fronts, etc, where mayo would be a constant, but his replacement won't be.
mayo's replacement will be the one guy coming off the field when everybody's healthy, so you'll just see a guy like skinner in there for 20 snaps, or whatever, with hightower maintaining his typical role.
if hightower is out again this week you probably have a chance at some production on a fill in, but that won't last when hightower returns.