Even if he is good at preseason, doesn't always mean he is ready for regular first team.
Pjaca was good at pirlo's preseason. Last preseason Kaio Jorge scored a hatrick in a match. That doesn't mean that they are ready for juve.
Its not that he will definitely isn't ready. The point is there's always some factors that only a coach can see. Even a newbie like pirlo know that Pjaca isn't juve material anymore. So relax and see what will happen.