He is not great, he is no horrible either. He is a weak starter or a strong back up.
Tannehill had a lot of potential and I genuinely believe he could have become a top 5 quarterback behind a decent line. Until his injury. After he came back, he was struggling in ways he wasn't previously, plus missing a year and a half in and of itself just naturally reduces the probability he could progress significantly.
At this point, if I were a coach, I'd want to see him perform in an actual game under pressure before giving him the job. Of course, the only way to see him in a situation like that is if gets the job first, so baring a Mariota injury or Tannehill *significantly* outperforming Mariota in practice week in and week out, I don't see him starting.