They're low because his usage is low because in those years we've had... Julian Edelman.
Amendola isn't going to replace Edelman's production, mainly because he isn't an Edelman-caliber player. But he'll replace some of it, and I suspect Mitchell and Cooks and likely even Burkhead will all play significant roles in replacing different dimensions of what Edelman does.
To be honest, I'm not overly worried here. Edelman is great, I love the guy and I don't think you can just replace what he does for this team. Not trying to diminish what he does at all. But we have a lot of players who can threaten the areas of the field that he threatens and can exploit the same sort of mismatches that he exploits. We don't have anyone that can get so reliably open within a second as he can, and that'll definitely hurt, but most of what we could do with him we can still do without him. It will always be a gut punch to lose one of the best players on the team, but we're more equipped to handle it with Edelman than we are with any of our other top-10 guys.