Harvin will be playing when he can. Actually, coming back in week 8-9 would be great if that is the earliest he can go.
Harvin is an exceptional freak of an athlete with a huge range of potential impact. Teams have to game plan for HIM on his own. He frequently turns screens and underneath catches into +10 gains, along with reverses or option plays.
You add that to an offense that already functions and my oh my. He can be on the field with ANY of the RBs and create so many openings for them.
Get him on the field ASAP. Force the opponent to deal with him. He could be in mid season form for the likely playoff run the Hawks will be him. No one has ANY footage of him in a Seahawks game. The opponent has absolutely no idea of what it will look like, he has only one season of football with a high level QB under center and that was Favre.