A lot depends on whether we qualify for Europe as well. If we do we simply have to get a couple more in to avoid completely running the likes of Raul and Jota into the ground.
A player like Cav provides an interesting conundrum - I personally don't think he'll ever be more than a barely okay option in the top half of the league (short-ish, short on pace and hardly ridiculously skilled to make up for it). At the same time, how much would a clear upgrade cost us, taking into account the risk of settling in issues? Probably a whole lot. And how much would that player actually play? The constraints of ffp make it an absolute necessity to make the most of inexpensive known quantities to make room for the top quality players able to take us to the next level.
The same thing could be said of Doc. His overall play is not quite good enough for where we want to be going, but the one trick this pony does, he does reliably and rather effectively. It must be tempting in many ways for Nuno to gamble on sticking with him and spend the £30m or more needed for an upgrade somwhere else.