Webster will get a chance soon enough. Three central defenders aren’t all going to stay fit and avoid suspensions for more than 6-8 games in all likelihood. Graham won’t have to drop anyone, it’ll just be the first injury or red card that costs someone a place. Once we’ve seen all four play and they’re all then available, that’s when Graham has to reveal his preference.
Dunk has only missed 2 league games in 2 seasons and Duffy 4 (3 for the red card against Palace). It will take a while to tot up enough yellows for a ban, so Webster might not get a chance quite as soon as you suggest unless Potter makes a change.