According to the Celtics, Horford was hit on the head during Monday's 110-107 win over the Atlanta Hawks and began showing symptoms of a potential concussion Wednesday morning.
Horford averaged 14.0 points, 6.8 rebounds and 5.0 assists per game in his first season with the Celtics and has followed up with averages of 14.6 points, 9.2 rebounds and 4.7 assists through 11 contests in 2017-18. His presence was a primary reason Boston finished with the best record in the Eastern Conference last season and is considered an NBA Finals threat again.
The 2007 first-round pick is a double-double threat who has averaged double-digit scoring every season of his career.
Boston can still rely on Marcus Morris, Aron Baynes and Semi Ojeleye to handle the frontcourt duties, although none of them make the same impact Horford does.