There's a lot of irony in criticising hyperbole and then comparing Garnacho to Ronaldo. The gulf in talent between those two is lightyears wide.
You're also severely underplaying how bad he's been. This isn't a bad run of form, he's been rubbish for the entire season, and was generally poor last season too. A more accurate assessment at the moment is that he previously had a run of good form, as his average level has been bad.
As for the potential to be great, what is it that you think he can be great at? He's one of our worst players for passing and crossing, one of the worst players in the league at dribbling, he's weak, isn't particularly fast, there's just so little there. He works hard, to his credit, but we need to be aiming much higher than that.