I believe this is Garrard's job to lose, just because of the way he practices and brings his talent to every practice consistently...I think the chances of a sudden dropoff happening are small. However, there's no way he's already the starter in the minds of the coaches.
I thought there was something interesting in the Peter King interview. King made it clear to Joe Philbin that he thinks Tannehill should sit. What's the rush? There are complex defenses he's not ready for, he talked about Oakland's defense as an example, Tannehill shouldn't take it as an insult if he doesn't start the season. He then described Joe Philbin's responding point and the way King described it, it almost seemed like Joe Philbin was making a counter-point against King's admonition not to start the rookie. He said that every rookie is different. Some guys like Matt Ryan can start right away while other guys like Aaron Rodgers sit and that ends up good for them.
This whole time I've been speculating that since it was Joe Philbin's experience that sitting Aaron Rodgers ended up working out beautifully, he would tend to see Tannehill within that context. But maybe he's thinking, if there's any rookie potentially ready to start right away, it's the one that has spent the last four years in the same offense under the same coaches that he's playing with as an NFL rookie.
There might be an actual desire to see Tannehill win this job, and if that's the case don't look for them to call the QB race until a little bit later...after the 2nd preseason game. They'll want to get Tannehill work and teaching as much as they can, and what will matter is how he looks against Garrard in the last week, not the first week.