After three years I still think that "Dia hampir seorang profesional" would be the most correct answer for this, although it is still not accepted (I have probably reported it more times than I remember).
Instead, I am required to answer "Dia hampir menjadi profesional", which at least seems to mean something quite different, namely "He has almost become a professional".
Maybe "Dia hampir seorang profesional" is not good Indonesian? But surely there must be some way also in Indonesian to say that one is "almost X" without saying that one has "almost become X", since these mean two different things.
Many things are "almost X" without ever "almost becoming X". This applies to being professional as well. Maybe I was earlier fully a professional, but now the requirements have become more strict, so I am now only "almost professional". In such a case (and many other cases) me being "almost professional" has nothing to do with me almost becoming one.