I really don't want Tannehill to turn into Alex Smith, Smith was forced into the starting lineup with the 49ers his rookie season, he was forced to learn the speed of the game under pressure. It didn't take him until last year to really get into the flow of things. And meanwhile Rodgers gets drafted by the Packers and just eases himself for a couple of years while he watches Favre, then when his time comes he's experienced and established chemistry and had game experience a few times.
Obviously there's a lot of Rookie QBs that do well their first season, like Cam Newton and Sam Bradford. I'm not saying it's impossible but I feel that it's best to let your QB ease into the game.