It depends on how much an injured player plays in the season when he was injured. You don't just automatically get a medical redshirt if you were 1/2, 3/4, etc of the way through the season when you got injured. It would be nice if a player that got injured toward the end of the season (that caused doubts to be raised about the injury with the NFL) to at least have the option to be able to come back for another year to prove he had recovered from the injury.
I think that is all anybody is saying when they argue about reasons why Tanner should redshirt this year. It's not that we think he's going to play another three years.