As a Turkish fan of both Celtics and Semih, I have to admit that I am really surprised and impressed with his recent performance with Celtics. Before the start of the season, I gave him zero chance to play more than 2 minutes in a game. It is not because he is a bad player but he is a rookie (+ a European rookie), he has a shoulder injury, he doesn't know English and lastly he doesn't know the defensive and offensive sets of NBA well and it will take some time to adapt it. As you know, European and NBA basketball is far away from each other.
However, he got the chance to play significant minutes in last games as most of the C's bigs were not available. I can't say he was the star of the games, he made lots of mistakes (especially at the first quarters) since he had the pressure of "what if I can't succeed" and the fear of getting early fouls in the defence. When he got used to play more minutes and concentrate the game without the pressure and fear I mentioned above, he showed that he can be a good backup player in short run especially for a 4th (or 5th big) and 2 round 60th pick.
It is too early to make keen judgements about him. I think he deserves a chance to show herself. He is a raw player, have lots of things to learn. He will be better within 1-2 years in my opinion.
Note : Sorry for the bad English, I'm not a native speaker. I hope I can express myself clearly.