Steve Bulpett of the Boston Herald and Adrian Wojnarowski of ESPN are also reporting that Horford will be looking for a three- or four-year deal on the open market.
Earlier on Tuesday, Wojnarowski reported that Horford was declining his $30.18-million player option for the 2019-20 season.
The 33-year-old centre averaged 13.6 points, 6.7 rebounds and 1.3 blocks on 53.5-per cent shooting from the field in 68 games for the Celtics last season, his 12th in the league and third in Boston.
Irving declined his $21.3-million option on June 12. Charania reported Tuesday that Irving will likely sign with the Brooklyn Nets this off-season.
The 27-year-old point guard averaged 23.8 points, 6.9 rebounds and five assists in 67 games last season, his second with the Celtics.