I wouldn't call for an engineer, if I was concerned, I would state something to the effect of 'while the roofing has been removed in that large area [hinting that it will take a large area], any concerns regarding the structure can be seen by removing an area of roof sheathing, check the structure, then re-install the roof sheathing and complete the repair' ... that's what the structural engineer would have to do to even see what might be wrong, if anything, in that area - they don't have x-ray vision either.
