Looks like a PF to me.
Is he good defensively? Looks like they couldn't even slow that team down
Good defensively? hard to tell how he is going to look like in the NBA. He has the BB IQ, hands, timing to pile up boxscore defensive stats, but i'm not sure how he is going to look like defensively guarding nba guards in space and nba centers. He is going to need to grow into his body and really improve his physique. He is so young, so it's possible.
I suppose Nathan is more fit to answer this question though.
Besiktas is not a good defensive team. Only 3 of their main rotation players (including Sengun) are taller than 6'4", and all of them are basically rookies. A lot of their games are shootouts, and they can win shootouts because they're a young team that thrives on pace and energy. Going into the series against Efes, they were averaging 92 ppg over their last 10 games, a very high average for 40-minute games (Gonzaga led the NCAA with 91ppg, for comparison).
Unfortunately for them, Efes is an even better offensive team...they led Euroleague with 62% true shooting and an offensive rating of 121.4, both incredible numbers. For comparison, the Nets led the NBA with 61% true shooting and a 118.3 offensive rating. That's obviously not to say that they're as talented as the Nets, just that they cut through the competition they faced in Europe about as easily as the Nets cut through NBA competition. And they're even more dominant against lower-level Turkish League competition like Besiktas.
The only real hope was for Efes to have a random cold-shooting game, but that never happened. They raced out to 30+ point first quarters in all three games, led by dynamic 6'5" combo guard Vasilje Micic who's generally considered to be the best guard in Europe right now, if not the best player overall.
In the last play here, you can see that Sengun is just a bit indecisive, unsure if he should help on Micic's drive or stick to his man (a 43% 3-point shooter).
He belatedly decides to help, and he *almost* gets the block, but gives up the and-1 instead, the worst possible outcome. I think this was his second foul, too, so this sent him to the bench.
That's just one play, obviously, but it sort of sums up how the more skilled Efes team preyed on the youth and inexperience of Besiktas. I don't think Sengun has any serious red flags defensively, and I think it's generally a good sign that he's trying to make plays even if he isn't always successful, but he's definitely a work in progress.
