On Friday, Celtics coach Brad Stevens said Horford had done more work and was being assessed day-to-day. He was held out of a loss to the Bucks.
Boston, which has lost three in a row, struggled without Horford, Aron Baynes and Marcus Morris on Friday. The loss inspired a team meeting afterward that kept the locker room closed for more than a half hour before eventually being opened to reporters.
The team is also in the midst of a challenging stretch, with five straight games coming against teams with .500 or better records to end 2018.
Horford, who is in his 12th NBA season, is averaging 12.4 points, 6.4 rebounds, 3.4 assists and a career-high 1.7 blocks in 22 games for the Celtics this season.
Morris, meanwhile, was listed as questionable for Sunday's game with right knee soreness. Baynes remains out for several weeks after breaking a bone in his hand in Wednesday's home loss to the Phoenix Suns.