Celtics legend Tommy Heinsohn, sidelined from NBC Sports Boston game broadcasts since late November while combating what his doctors believe might be a sleep disorder, will return to the airwaves Friday night when he phones in during the second quarter of Boston’s game against the visiting Dallas Mavericks (NBC Sports Boston, 8 p.m.).

The 84-year-old Heinsohn is set to be discharged from a local inpatient rehab facility on Saturday and his doctors have targeted sleep apnea as the likely cause of the health woes that have forced his absence from game broadcasts.

"I was nursing it along for quite a while then it really started to where I couldn’t get to the games,” Heinsohn said in a phone conversation on Thursday. "But I’m rehabbing right now. They’ve ruled out all [big concerns] -- my heart is real strong, the lungs are clear, nothing wrong with my stomach, and we’re down to, perhaps, sleep apnea. I gotta go through the tests. They've put a mask on me here at the rehab.”

Heinsohn plans to spend time at a sleep center after leaving rehab. He’s uncertain when exactly he’ll be back at TD Garden for broadcasts but is eager to do such once he’s got his health concerns under control.

Asked about early trials with a sleep mask to combat the apnea, Heinsohn let out his familiar cackle but said he’s seen immediate results.

“[The mask has] stopped me from waking up four times per night. I’m feeling refreshed in the morning. The day before Christmas, my granddaughter gave me one of those watches with the NitFit or whatever the hell they call them,” said Heinsohn, referencing an exercise and sleep tracking FitBit. "She gave it to me and then the next day she was going to tie it into my iPad and I found out she tracked my sleep from the night before, OK? I slept 2 hours and 7 minutes, and I thought I slept 8 hours."

Heinsohn, his energy sapped, initially sought treatment at New England Baptist Hospital where a battery of tests ruled out more serious concerns. He moved over to New England Rehab in Woburn and gushed about the way the staff has got him moving again.

"I feel like I’ve been at Red Auerbach’s training camp for Chrissake,” said Heinsohn. "I put my foot on the floor out of bed and every muscle in my body is aching now from doing exercises. They’ve got me walking around this place, which is pretty good right now. That part is working.”

Heinsohn is touched by the outpouring of support from fans who have noted his absence from broadcasts. He is excited to chat with his broadcast partner of 37 years, Mike Gorman, during Friday’s telecast. And he’s got a month’s worth of basketball opinions to deliver.

"It’s not that I’ve been hiding,” said Heinsohn. "I’ve been watching all the games.”

Heinsohn was eager to offer his thoughts on all things Celtics, particularly in the aftermath of Wednesday’s Kyrie Irving-less win over the Timberwolves.

“Gordon Hayward [on Wednesday] night was fabulous,” said Heinsohn. "What I was hoping he would do is find out that, coming off the bench, he should take his best shot, the shot he has the most confidence in, as it develops in this offense. Instead of taking it to the rim, 2 inches away, and kicking it to the corner. He’s a jump shooter — 15- to 18-foot jump shooter, it’s a significant part of the game, the defenses are giving that away now.”

Heinsohn also liked what he saw from Terry Rozier, who elevated to a starting role in place of Irving.

“Listen, Terry Rozier’s a hell of a player. Kyrie is a great, great player. Rozier’s never going to be a great, great player but he’s going to be a hell of a player for the Celtics, who I hope will be able to keep him,” said Heinsohn. "But people expect him to produce the same results he did when Kyrie was out last year, while coming off the bench and playing limited minutes with second-line guys. And there’s no chemistry. I think Terry Rozier is playing great.”

As for Boston’s uneven play for much of the first half of the season, Heinsohn said the month of January will go a long way towards showing exactly what this team is capable of this season.

"Now we should be judging this team — what’s happening now for the next month because they’re playing a lot of home games,” said Heinsohn. "To me, the whole training camp, which was what, eight days? Not even? Four days before they played the first preseason game? How the hell do you put a team together when you’re trying to incorporate two hurt players from last season? I mean, please. [Coach Brad Stevens] isn’t Houdini, you know.

“And one of the things [the Celtics have had to] deal with is different lineups, because of injuries, or minute restrictions. It’s like cooking a meal and you forget to put salt in it or pepper. You’re missing something and it doesn’t taste quite the same. That’s what’s going on here. How many times have they had what you’d call the 'regular' lineup?”

Like the Celtics, those of us here at NBC Sports Boston are eager to have our regular lineup back on the court as well. Heinsohn sounded optimistic that doctors have identified the culprit that’s kept him off the air and is eager to combat it.

Said Heinsohn: "We’re on the road to finding out why I can’t sleep.”

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