Has proven he has the quality, albeit in a lower league. In my opinion, Mitrovic is one of the few cases where I've disagreed with Rafa. I think he should've seen more game time earlier in the season, especially when it was clear that Gayle and Joselu couldn't get a goal to save their lives.
The question; would you have him back for next season? I'm sure he'd demand being the main striker.
I suppose if Rafa doesn't rate him, there's no point in hanging onto him.