YES

   1: f(a(),x1) -> f(a(),g(x1))
   2: a() -> b()
   3: g(x1) -> x1

@Rule Labeling
--- R
   1: f(a(),x1) -> f(a(),g(x1))
   2: a() -> b()
   3: g(x1) -> x1

--- S
   1: f(a(),x1) -> f(a(),g(x1))
   2: a() -> b()
   3: g(x1) -> x1