With Gronkowski being as limited as he is, it's surprising they have been suiting him up. To see such an important piece of this team limping coming out of his breaks is pretty scary. This guy is important and he needs to get his body right.
@supafly and @SVN already posted the most important parts I wanted to address.
But to summarise.. Gronk is being eased back slowly into the game without exposing him to dangerous situations in the passing game. Until now he has essentially only contributed as a blocker and I think that was a masterstroke as he is now definitely used to the normal wear and tear of games in terms of conditioning.
Given our great start into the season under the circumstances (3:1) it is a good decision to consider the big picture over a division game at home with QB3 starting.
Personally, I expect him to be slowly moved back into the centre of the offense agains the Browns and be back to his usual role against the Bengals. In other words, no reason to be concerned unless he remains in his role of inline blocker for the next 2 games with Brady at the helm.