Emmons and Arrington had a full season between the two of them. While it seems like it would be a major blow to the defense, they have played without them and can make do. Cutting Petitgout really hurts their depth (especially considering that Whitfield retired as well), although their actual starting OL will still be OK. They might be able to pick up a decent cheap OL backup? I agree with the assessment of the secondary in this thread - it needs help. Madison & McQuarters both looked old and overmatched. I hope Manning lives up to his pedigree soon. Otherwise, he'll go the route of Dave Brown. Sadly, Kerry Collins might end up the most successful QB of the Giants' last 10-12 years.
EDIT: Grammar