Versatility, as in being able to play any of the supporting positions behind a striker, does seem to be a prerequisite for attacking midfielders now. Makes sense given how long it's been since the death of 4-4-2.
I think that Sigurdsson is a perfectly serviceable player, but for the money spent we should have been able to find much, much better. I've been disappointed by his lack of craft or guile, and, like Williams, it seems as though his leadership skills just don't translate at a bigger club.
The annoying part is that he has the technique and skill, just no balls or heart to see it through