It all depends on where the Patriots stand. In all likelihood, by mid-December the Patriots will have sewn up the AFC East, but also will be out of contention for a bye. Houston is two games up and the Ravens are half a game up. The Pats have four tough games in a row, traveling to the Jets and Dolphins, then hosting the Texans and Niners. Even if they go 2-2 there, most likely Denver and/or Baltimore will have leapfrogged them.
If the Pats have indeed locked up the division but are out of the race for a bye, it's best to just rest him until he feels 100%. I know there will be some rust, but he can still train with the forearm injury. He'll have his legs under him and it will just be getting timing with Brady. That is admittedly very important, but to some extent it's like riding a bicycle with these guys - Brady knows how to get Gronk the ball where he likes it.