Robles is no where near that class.
There's a few levels imo.
De Gea, Lloris, Courtois, Cech
Schmeichel, Forster, Butland
Mignolet, Karius, Begovic, Robles, Pickford, Cabellaro, Bravo, Hennessey etc.
We need to move up a level.
The top keepers are at CL clubs and unlikely to join clubs outside of the CL. In fact, there just simply aren't that many 'top' keepers about.
So we have to be realistic, and go for a player in that second level. Prem experience in such a key position is, IMO, a real bonus as well, which is why I think we may well make a move for Forster or Schmeichel (Pickford is another option, and given his age he's only going to improve).
Alternatives are out there of course, but it's not a position to take a risk with. Leno, Sommer are both good 'second-tier' keepers that'd be great signings - though Karius looked like he would be too (albeit he's had a crap defence in front of him).