So what do the Boston Celtics look like when mostly whole? Like a whole lot of trouble for the Miami Heat.

With Marcus Smart and Al Horford back in their mix after missing Tuesday night’s series-opening loss in these best-of-seven Eastern Conference finals, the Celtics during one first-half stretch outscored the Heat by 40 points in surging to a 127-102 victory Thursday night at FTX Arena.

“They came out and hit us in the mouth,” Heat center Bam Adebayo said, “and we didn’t know how to respond.”

In suffering their first home loss this postseason after seven such victories, the Heat fell behind by 29 in the first half and never regained their footing.

“We just have to figure some things out,” Heat coach Erik Spoelstra said.

While it was Jayson Tatum leading the way for the Celtics with 27 points, it also was Smart and Jaylen Brown each scoring 24 for Boston, with Horford providing stability on both ends. Smart added 12 assists and nine rebounds.

“It was great to have the vets back obviously, calming presence there,” Celtics coach Ime Udoka said.

Jimmy Butler, coming off Tuesday’s 41-point performance in the 118-107 Game 1 win, this time led the Heat with 29, sitting out the fourth quarter, with matters decided by then.

While the Celtics got back to their primary rotation, the Heat were without Kyle Lowry for a fourth consecutive game, due to a hamstring strain. The Heat’s only comment about Lowry after the game was that the veteran point guard “was making progress.”

Now with homecourt advantage, the Celtics host the series’ next two games, starting at 8:30 p.m. Saturday at TD Garden.

Five Degrees of Heat from Thursday’s game:

1. Over early: The Celtics’ 70-45 lead at the intermission was the largest on the road in their playoff history.

Tatum scored 20 in the first half, with Brown adding 15 over the first two periods for Boston.

The Celtics had the Heat out of sorts to the degree that Spoelstra’s team closed the first half with nine assists and eight turnovers.

“It wasn’t surprising to see how well we guarded with our guys back,” Udoka said.

And all of that came after the Heat jumped to an 18-8 lead at the outset.

“You definitely have to credit Boston,” Spoelstra said. “It’s not that they just came in with great urgency and physicality – they did have that, for sure – they also played well.”

2. The hard way: Smart took the defensive assignment on Butler from the outset and initially made things considerably more challenging than when Butler scored his 41 in the series opener.

After attempting 18 free throws in Game 1, Butler did not get to the line until 7:24 remained in the second quarter.

Butler then scored 16 in the third quarter, but by that point it largely was window dressing, closing 11 of 18 from the field and 6 of 8 from the line.

3. Tucker ailing: As if things weren’t bad enough, the Heat lost starting power forward P.J. Tucker for the night in the third quarter with a knee contusion.

That was after Tucker briefly had to leave Game 1 with an ankle sprain.

Tucker, 37, scored five points in Thursday night’s opening minute and then did not score again.

He played 21:14, finishing 2 of 6 from the field, with four rebounds.

“Ask him, he says he’s good to go,” Soelstra said.

4. Why not?: It reached the point where Spoelstra turned to Duncan Robinson with 2:20 left in the third quarter.

That came after Robinson was held out of Tuesday’s series opener, as well as three of the previous six games.

With Caleb Martin continuing his postseason struggles, rotation adjustments could be a consideration.

More significant, of course, is if/when Lowry returns.

“Look,” Spoelstra said, “you get to this point of a conference finals, you just have very good teams. You have teams without many weaknesses. It’s great competition. This only counts as one.”

5. Two back: The Celtics had Smart and Horford back in their mix and back in their starting lineup after both missed Tuesday night’s series opener, Smart with a foot sprain, Horford in NBA heath-and-safety protocols.

Missing, though, for Boston was guard Derrick White, who left the team for the birth of his child.

White, who had started Tuesday in place of Smart, is expected back for Saturday night’s Game 3 at TD Garden.

“It came sooner than expected,” Udoka said of White’s plans, “but we support our guys always in that situation.”

The Celtics acquired White at midseason from the San Antonio Spurs in exchange for former Heat guard Josh Richardson.

With Horford, it was a waiting game from the outset.

“He was not feeling ill,” Udoka said Thursday of Horford. “He’s feeling fine. He’s OK. It’s something that was unexpected.”

Udoka declined to identify why Horford entered testing.

“He passed all the protocols, passed all the tests,” he said pregame. “I’m not a doctor, but he had to pass a certain amount of tests.”

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