Al Horford signed a four-year, $109 million contract with the Sixers this summer, ending his three-year run with the Boston Celtics.

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, Al 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.

“When we started with the team trying to come up with things and we couldn’t agree on certain numbers, that’s when I decided, you know what, I’m going to have to open my free agency,” Horford told the Boston Herald. “I believe not only that I am worth a certain amount of money, but also I want to be in a position that I have a really good opportunity to win now. You know, my window is now. That’s how I feel.

“I had to think about what made sense for me and some of the things that I wanted to accomplish. They were three really good years for me there, but we just fell short of that goal of raising that banner. I had to make a head-on decision, and it came down to the opportunity for me to compete for a championship, but also to maximize financially.”

Horford 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 now 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.