Horford could re-sign with the Celtics this summer, according to ESPN.
Horford, 33, averaged 13.6 points and 6.7 rebounds last season. He was named to the All-Defensive Second Team in 2018.
The veteran big man was drafted in 2007 after winning two national championships at the University of Florida. He was a four-time All-Star with the Hawks from 2007 to 2016.
Boston went 49-33 last season. The Celtics have not made the NBA Finals since the 2009-10 season.