Philadelphia 76ers big man Al Horford won't play on Thursday when the Sixers take on the Boston Celtics in Boston.

Horford spent the previous three seasons in Boston before signing with the Sixers this past offseason:

Horford signed a four-year, $109 million contract with the Sixers. He averaged 13.6 points, 6.7 rebounds and 4.2 assists last season with Boston while shooting 53.5 percent from the field, 36.0 percent from beyond the arc and 82.1 percent from the free-throw line.

The Sixers are 18-7 on the season. Horford is putting up 13.8 points, 6.8 rebounds and 3.9 assists. He's shooting 48.0 percent overall, 36.6 percent from the 3-point line and 67.6 percent from the charity stripe.

The Celtics underachieved as a team last season and were dysfunctional. Horford, one of the leaders in Boston, knew that big changes needed to be made if the Celtics wanted to run it back and compete for a championship.

While changes were made in Boston with the departures of Kyrie Irving and Terry Rozier, Horford just felt like playing with the Sixers gave him the best opportunity to win a championship. It also helped that Philly was willing to pay him a contract he was looking for.

The Sixers have four elite defenders in Horford, Joel Embiid, Ben Simmons and Josh Richardson. Philly acquired Richardson from the Miami Heat in the Jimmy Butler sign-and-trade deal.