Anunoby would be an awesome PF to have on this team next to Tatum & Jaylen on the wings.
Is he actually a PF? He is list at 6'-7", 232 lbs. I am not sure that he is the answer to our PF issue.
As for trading him for Turner, seems Indy wins that by a mile.
Anunoby is a forward. He can play both. It is not one or the other. He is a combo forward.
What allows him play the PF the most is his exceptional strength. The guy is phenomenally strong. This is what most allows him to matchup against guys who are taller than him and hold his own. They can't bully / overpower him.
He has principally played the SF position in Toronto but he has played minutes at PF as well and he has played very well there. The main reason he has not played more there is because of Pascal Siakam.
One of the main benefits for Anunoby of playing PF instead of SF is his skills relative to his position / opponent in terms of ball-handling, passing and shot-creation measure higher against the lower skilled PFs than higher (perimeter) skilled SFs. So your team gets a boost in offensive efficiency / output and he individually gets a boost in his offensive output while not hurting you defensively because of his incredible physical attributes at 6-7 and tenacity defensively.
On the Celtics, Anunoby would primarily play PF and do the dirty work so that Tatum and Jaylen do not have to. Anunoby would guard all the big time PFs on opposing teams. He and Tatum are switchable enough defensively that against weaker PFs that they could switch matchups so that Anunoby could defend a top SF (say to defend Jimmy Butler while Tatum defends PJ Tucker down in Miami). Anunoby is one of the toughest defensive forwards in the league.