A couple positives from this after cooling off:
1. It's not a season ending injury and 2. This gives his back/hip a few more weeks to heal without getting banged up in practice.
To me, Gronk isn't the type of guy that is going to need a ton of time to get re-acclimated to the offense. It's not like he's Welker out there running precise routes that depend largely on timing with Brady, the man is a massive target with phenomenal hands. He's going to get his on day 1 when he gets back.