To prove my point here ar a couple of photos which faithfully portray the site to the modern visitor. Contrast them with Stukeley's sketch.
In this first photo the two stones can be seen to stand in a shallow hollow. This barely discernible depression in the field is all that remains of what was originally a substantial enclosure. Even Stukeley does not add such a feature to his sketch though so maybe it was eroded long before the stones were vandalised. We know of the earthwork, (resembling a surrounding ditch and bank like a small henge) only through archaeological investigations of the site.
This photo is taken looking through the 'entrance' in the circle's surrounding bank. It is now hard to make it out at all, from the field. The two remaining stones are in the bank on the far side of the circle.