I'd think there'd be two easy assumptions, and Glastonbury is a bit of a marker:
Bono has done something relatively simple that might not necessarily require surgery as the only option, but for someone wanting to be up and running quickly, that is the best option. But it is in reality a relatively minor/generic injury that with the kind of care he'll be receiving, won't take too long to recover from. They take a month off, give or take, meaning they drop the UT/CA/CO gigs and return for Seattle on June 20. That gives a month of recovery, and confines the rescheduling to one logistically easy area - no jumping around the country - that might possibly be squeezed in at the end of this leg, before the European leg. It also means they don't have to ditch Glastonbury - a gig of huge importance that can't be rescheduled - and have a couple of warm up gigs before it. All of this is a massive hassle, but not a huge drama.
Option 2 is really just that he's done something that is looking to keep him out beyond Glastonbury (be it 6 weeks or 6 months), in which case, I'd think the whole leg is likely in jeopardy and probably will be entirely canned and moved to next summer - after Europe, and perhaps even after a Southern Hemisphere swing sometime between November-March.
Either way, I think Glastonbury is the marker. I think you'll hear about the next 5 or so gigs getting formally canned in the next couple of days, and soon enough the strongest rumours will start coming from the Glastonbury camp, not the U2 camp. Whether they are staying in, whether they are canning it, or whether a big replacement is getting lined up, etc. If we get wind of Glastonbury definitely being off, then I think that's a pretty good sign that the whole US leg could go.