Sengun playing against Greece right now. Watching him he has a great feel for the game but definitely lacking in athleticism. Really limited vert and looks to be more 6’9” than 6’11”.
He played ok in limited minutes, second most efficient player on his team with 12 points, 4 rebounds, and 1 assist. There was also I play saw posted on twitter saying that he looks like he's stuck on mud on a defensive play. Sengun has a bad habit of trying to jump a play/lunge at a defender, which is exactly what happened in that play. He lunged forward and the offensive player attacked him while his body/hips were twisted in the opposite direction.
Sengun has a surprising vertical, he can actually get up when he fully gathers, his north-south speed is pretty good while also having a good handle for his size, but obviously he is not the quickest laterally. Could the lateral quickness be a problem? Maybe, but he's not hopeless on defense, he has bad habits he needs to work on that will help his perimeter defense.
I liken him to an Al Horford more than a Sabonis. Sengun and Horford have roughly the same measurements coming into the league and just watching them both play they are similar athletically. Horford doesn't blow you away athletically, he is just really fundamentally sound on defense along with being able to read, react, and recover quickly. Sengun averaged 2.1 blocks, and 1.7 steals per 36, so he has the ability to read and react on defense fairly well, and he isn't bad at post defense outside of biting too much on pump fakes. So he does have potential on defense, and has statistically shown it. He has proven to be an extremly hard worker, and he has said in an interview that he knows he has to work on his defense, so it's really up to him if he can figure it out.