This is a GREAT pickup.
I have always liked Tannehill and thought he could have been one of the better QBs in league if not for injuries. While I think Mariota has more talent, Tannehill performs better regularly, more consistently.
My biggest frustration with Mariota is not necessarily that he is injury prone, but that he doesn't seem to be able to play through injuries like most QBs do. Any little boo boo and he is out. Also, he is infuriatingly inconsistent. He will show flashes, have a great game and then crap the bed for 2 games.
If I am JRob/Vrable, I start Tannehill. I feel he gives the Titans the best chance to CONSISTENTLY play well and if he gets hurt, Mariota is a fantastic backup, who is good for only a few good games a year, so maybe its the perfect situation.
Tannehill with the weapons we have (Davis, Walker, Lewis and Humph) will throw for 4000 yards/20+ TDs easily.