"It was scary, but he's doing well," Ainge's son, Austin, told FOXSports.com. "We're relieved because all of the tests came back well. We expect him to be up and around in the next day or so. It's under control."

Ainge, who turned 50 last month, suffered the heart attack at roughly 5 a.m. ET while he was sleeping. The team issued a statement Thursday saying that Ainge was "recovering nicely" but is expected to remain at Massachusetts General for a few days.

"All of Celtics Nation is thankful that Danny got incredible medical care so quickly and so effectively," Celtics owner Wyc Grousbeck said. "We wish him, Michelle and the kids all the best and know he will be back with us very soon."

A member of two Celtics championship teams in the 1980s with the original Big Three of Larry Bird, Kevin McHale and Robert Parish, Ainge was an NBA All-Star in 1988.

As general manager, Ainge helped the franchise raise its record 17th NBA championship last season by assembling a new Big Three, acquiring Kevin Garnett and Ray Allen to join Celtics veteran Paul Pierce. Ainge was named 2007-08 NBA executive of the year and in October was rewarded with a contract extension and the title of president of basketball operations.

The Celtics are scheduled to begin the playoffs on Saturday against the Chicago Bulls.

Ainge missed Wednesday night's regular-season finale because he wasn't feeling well. He woke up in the morning with chest pains.

Ainge was a standout basketball player at Brigham Young, winning the Wooden Award awarded to the nation's top college player in 1981. He also played three seasons of pro baseball, batting .269 over 211 games for the Toronto Blue Jays.

Information from The Associated Press was used in this report.