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aren't the only ones talking about Tillman as a potential ace. BA has him as one of the top 2-3 prospects in the minors, as does Goldstein. You're also underrating where he was at in the minds of talent evaluators when he was dealt. He was universally written about as a breakout candidate, having come out of High Desert with a non-stratospheric ERA and a very good K/BB rate.
I'd be surprised if he turns into a Cy Young candidate as well, but he's got that kind of upside. I'd be more shocked if he turns into anything but an above-average starter for any reason other than injury at this point, though.