BOSTON (AP) — The Celtics have traded point guard Kemba Walker to Oklahoma City for forward Al Horford. It’s the first major move since Brad Stevens moved from the Boston bench to the front office. The Celtics will also send the No. 16 overall draft pick to the Thunder and receive center Moses Brown. Horford is returning to Boston and gives the Celtics the frontcourt presence they have lacked since he left two seasons ago. Walker helped Boston reach the Eastern Conference finals in the NBA bubble last season, but knee injuries limited him to 43 games this year.