There are still four or five games before RR returns, so lots of time for things to change. But assuming TH continues to play at his current level, I would support starting TH initially, ensuring RR gets gradually increasing amounts of playing time each week. When RR's fully healthy and has seen a fair bit of carefully managed game action in relief, he goes back to being No. 1. You don't lose your job because of injury, you lose it because of performance, and his most recent performances were arguably as good or better than TH has been this season.

That said, you start talking to TH right now about an extension with the goal of getting him signed beyond this season to a contract that would work if he is the undisputed No. 1 QB with inexperienced backups, OR he is one of two starting-calibre QBs (i.e. in the $300k range). And you have a heart-to-heart with RR to say we want you back in this offence that was made for your skill set, but it will be in a two-QB system and your salary will need to reflect that. If he says absolutely not, get TH signed and let RR walk or change his mind later. (And I reserve the right to change MY mind on this over the next month or two.)