Using subs to replace symbolic expressions for nonparametric derivatives

Matlab's notation for the symbolic derivative of an unspecified function is not easy to work with. Eg., after symbolic differentiation of VFD12(exxFst1,exxSnd11(exxFst1)), I end up with a symbolic expression like (D([2], VFD12)(exxFst1, exxSnd11F(exxFst1). Later, when I have a functional form for VFD12, I want to use subs to replace the place-holder with the newly defined expression. But subs doesn't recognize a ghastly expression like (D([2], VFD12)(exxFst1, exxSnd11F(exxFst1). I can convert my symbolic expression to a character string and use strrep, but that's extremely cumbersome. Any advise would be appreciated!