Proved?
Matusz had 10 or so very good starts at the end of 2010 and has otherwise been dreadful. It's not like he had it all figured out. Baseball history is littered with pitchers who put together an excellent 1/3 of a season.
Yeah, I think "proved" is right. Look, it's not like he was a 1-pitch wonder and then everybody figured him out. He had 4 pitches and he was good with any 2 of them. In his first ST right out of college, DT took pitches away from him and he still got everybody out.
Now, to be fair about it, as DuffMan and Mike B have pointed out, significant parts of his decline may be due to him being a knucklehead who won't listen. But it's also true that he had a different PC every time he turned around. How much of it is due to which factors? Beats me.
But, while I truly don't know how much of his decline to attribute to what, I do feel quite sure saying he was indeed more-than-fine right out of the box, the problem is what happened later. It's not that he was just fooling everybody at first. He really was a remarkably complete SP before the O's happened to him... or before he happened to himself, or whatever it was...