WALTHAM — Jeff Green has spent the past few months downplaying his heart surgery, taking his career-threatening injury in stride after missing all of last season.

On Saturday at the Sports Authority Training Center, Green participated in his first official practice in 16 months. Though he signed a four-year contract over the summer to remain a Celtic, he felt his return didn’t become official until his first workout.

“It felt good, I was happy to be back on the floor,” said Green, who is expected to be the first forward off the bench. “People make a big deal out of practice but I was glad to have a practice under my belt and get going.

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“I was trying not to smile too much during practice. I had to keep a straight face, but it’s hard, man. I let some emotions go, telling all the players I was happy to be back, just encouraging them, just talking a lot for no reason. I’m glad to be back on the floor.”

Green reported to training camp last season but his initial physical raised concerns. After two weeks of uncertainty, he visited a heart specialist and an aortic aneurysm was detected. Green had surgery at the Cleveland Clinic Jan. 9 and made frequent trips to Boston to visit with teammates and attend games.

“It meant a lot, man, having [Kevin Garnett] to my left, Paul [Pierce] to my right, playing with [Rajon] Rondo, [Jason Terry], it’s a blessing just being in this position again to play basketball,” he said.

“I’ve gone through the tough times. I beat it and I am in great condition and my game is getting right and I’m just happy to play basketball. The surgery is behind me and good times are in front of me.”

Already shaping up

Doc Rivers’s biggest issue before last season’s lockout-compressed training camp was conditioning. Rivers said he could tell during the first few workouts that his team was out of shape. After Saturday’s opening workout, he is much more confident.

“It was a good day,” said Rivers. “We had a pretty good practice. Some guys are in great shape, some guys are in average shape, but overall with all the numbers, I thought it was a good practice.

“I think it’s easier when you have a date when you know you have to start and you’re allowed to work out until that date.”

Pierce was nursing a sore heel entering last year’s training camp, an ailment he said didn’t completely heal until after the All-Star break.

“It’s definitely an advantage to be able to come out here and be already in shape,” Pierce said. “Last year, injuries happened coming into training camp, so I get off to a better start this year.

“Hopefully I can get through training camp, get through preseason without any injuries. I think it’s important that we start healthy and I think it’s important we get off to a good start.

“I think with the team we got, with the added depth, with the youth mixed in with the experience, it’s going to allow us to play better, in back-to-back situations, in situations where we didn’t play well last year.”

Stretch run

The Celtics worked out for about 2 ½ hours Saturday and ended the session with stretching, something they did not do last season. Garnett ribbed teammates during the stretching, giving rookie Jared Sullinger a hard time about not completely bending his knees . . . All 17 players participated in practice, including invitees Micah Downs (wearing No. 55) and Rob Kurz (No. 57). Darko Milicic is wearing No. 99 and Jason Collins No. 98.