Al Horford became a Boston Celtic on Friday.

The long-time Hawks center, a cornerstone of the organizational foundation for nine seasons, had his introductory news conference. The official end of his time with the Hawks after he agreed to a four-year, $113 million contract as an unrestricted free agent last week.

Horford was asked about his decision to leave Atlanta.

“It was a very difficult decision for me,” he said. “I had a great situation in Atlanta, great years. I have a lot of respect for the organization and the fans there. Looking here and what’s ahead, the vision I see here, not only with the owners, the GM and coach (Brad) Stevens. I liked the direction I saw when I met with them. I felt comfortable. I felt confident in the potential and what can be.

“I’m encouraged by the group of players who are here. A lot of young, hard-working, talented players. Playing in (last season’s first-round) series, I saw the passion that they played with and how much better they got as a team throughout the season. It’s something that excites me to be a part of this.”

Hawks president of basketball operations/head coach Mike Budenholzer thanked Horford for his time with the team in a statement.

“On behalf of the Hawks organization, I would like to thank Al Horford for his many contributions on the court and in the community,” Budenholzer’s statement read.

“He was an integral part of one of the most successful runs in franchise history, including nine straight playoff appearances after his arrival. The character he displayed on the court, in the locker room, and in the community gave our entire organization and city a leader that embodied unselfishness and professionalism.

“We respect Al’s decision and are grateful for his nine years with our organization and community.”

Horford will wear No. 42 in Boston. His No. 15 is retired in Boston, as it belonged to Hall of Famer Tommy Heinsohn.

It was Heinsohn who said Horford was not a “great” player during their Eastern Conference first-round playoff series last season.