"We all know we have to take a real big step forward as a basketball team," Spoelstra added.
The next chance for that "big step" comes Thursday, when the Heat visit Chicago.
Down by 18 in the first half, Miami cut the deficit to 65-57 at halftime. James closed it with a 3, punching the air as the shot dropped.
Even with that, every moment of the second half was an uphill battle.
And the Celtics weren't being caught.
Miami got within five twice in the third, with Rondo answering both times to make it a three-possession game again. The Celtics held the Heat to 36 percent shooting in the quarter, keeping the eight-point lead entering the fourth at 89-81.
The Heat had two chances to tie early in the fourth, after starting the period on a 7-0 run. The first of those slipped away when Bosh failed to finish off a three-point play opportunity by missing from the line. The next came when Udonis Haslem missed from the left baseline with 9:17 left, and Boston's answer that time was absolute.
Allen made a catch-and-shoot 3 for a 94-89 lead, and then Garnett just took over.
"When they shoot like that, it's going to be tough to beat them," Wade said. "Obviously, we can always do things better. But give them credit."
NOTES: Alex Terrio, a fan wearing a Heat T-shirt, won a 2012 Kia Optima for making a halfcourt shot in between the third and fourth quarters. "Shock," Terrio said. ... James had 11 points in the third quarter for the third consecutive game. ... Hall of Fame quarterbacks Dan Marino and Jim Kelly were at the game, as was Chicago Bears star Julius Peppers.
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