It's unlikely that Gronk will play week one. Last year he took his time coming back from multilple surgeries on the advice of his inner circle, which the Pats were not happy about. After an ACL he is likely to be even more cautious. The Pats know they will win the division as long as they have Brady, which has happened every year since 2002. Therefore, they don't need to rush Gronk back, they need him at full speed for the post season.
But don't kid yourself about this game. The Pats have made it to the AFC Championship game each of the past two years without Gronk. They fortified their weakest link (pass defense) this off season, and will be tougher than ever.