Of course it is, but it's not like we're talking about Bob Sanders, here. He's missed 23 games since 2009 compared to Welker's 3, but he's also five years younger than Welker and has never had a devastating knee injury. Welker clearly has a better record when it comes to durability, but Amendola's injuries (the ones I'm aware of) have all seemed like pretty freak things, like the clavicle that almost pierced his aorta.
I don't think Amendola's going to come into Foxboro and put up the same numbers Welker did, but I don't think this is some awful signing, either.
EDIT: Oh, and check this out: evidently Amendola signed with the Patriots
before Welker did with the Broncos. So apparently this wasn't just some reaction signing by New England; considering the deal they offered him and the one they offered Welker, it looks like they wanted Amendola more, for some reason.