We know this from what? His stats last year in his one start were as good as Bowman in that horrible QB rotation. Less completion percentage, but higher TD and I believe lower INT. Since then he's not gotten first team reps. If you want to base your opinion on a couple series against a good Utah defense when he came in cold after not taking any snaps with the 1s, then that seems a bit short sighted. I'm not saying he's the next great OSU QB, but frankly, we don't know.
All we do know is that Bowman isn't going to improve, and he's not good enough. He wasn't good last year, but having a working running game and a better defense disguised many flaws. Others thought that he was very good with ball security because he threw it away so much, completely ignoring the TD/INT stats.
While at it, I'm very disappointed in Tim Rattay if Bowman is a symbol of his ability to coach for improvement.