BOSTON — There was a moment in the past month when Marcus Smart wasn’t sure where he’d be playing next season.

A day after signing a four-year deal to remain in the only NBA jersey he’s ever played in, Smart said he’s focused on doing what he can to help the Boston Celtics win their 18th title.

“I’m ecstatic. This is a blessing,” Smart said Friday.

After being in limbo since the start of free agency, Smart cemented a deal Thursday with the Celtics. A person with knowledge of the agreement said Smart will be paid $52 million over four years.

Boston returns a 6-foot-4 defensive cornerstone who developed a reputation for toughness and doing little things that help the Celts win. Smart is also the longest-tenured player on the roster.

The confidence that General Manager Danny Ainge shows isn’t lost on Smart, who acknowledged he knew he was entering a tough free-agent market this summer.

“To be honest, I didn’t know where I was going to end up. I was just enjoying this whole process,” he said. “It is a business so things aren’t perfect. That’s why it’s called negotiations. You guys come together and you finally agree on something. We both agreed. Boston loves me and I love Boston. Boston wants me here and I want to be here. I am here. So we made it work.”

Smart’s return also means Boston will bring back the core of the team that won 55 games and reached the Eastern Conference finals.

Smart was a huge part of the run, stepping in to start after returning from thumb surgery late in the first round of the playoffs. Boston pushed LeBron James’ Cleveland Cavaliers to seven games in the conference finals.

Now James is in the West with the Lakers, and given the emergence of Jaylen Brown and Jayson Tatum, Smart said he doesn’t see any reason why the Celtics shouldn’t be a favorite to come out of the East.

“We demonstrated the talent and ability we had to do that last year with a few missing pieces. With those missing pieces back in action, I think it really makes it hard for teams,” Smart said.

The signing also will allow Smart to shift his attention back to his mother, 63-year-old Camellia Smart, who continues to undergo treatment for bone marrow cancer. He’s been with her in Texas since the season ended, and said she’s stable and doing well.

“She’s hanging in there,” Smart said. “This is a hard time. At the same time it’s an exciting time for my family. … Right now she’s doing great.”