I think Yoshi is physically more gifted than Sirles, but "the best player" is often the guy who knows the playbook best. Yoshi may be the second coming of Orlando Pace, but if he doesn't know who to block, when, and why, he's not "better than" Sirles.
I have no inside information and this isn't a knock on Sirles, but from what I understand the only thing holding Yoshi back is learning the playbook. Once he gets that down enough, I expect him to start the rest of the year.
A good example of this is DeJon Gomes last year. All the talent in the world, but didn't start until the Texas Tech game because he was still learning.