Miami ? Alonzo Mourning was supposed to be an insurance policy for the Miami Heat, someone to give them a few good minutes off the bench and take some rebounding and defensive responsibilities off Shaquille O’Neal’s mammoth shoulders.

On Tuesday night, Mourning was much, much more for the Heat, who took a 2-0 lead in their Eastern Conference quarterfinal series with the New Jersey Nets.

Mourning, who wanted out of New Jersey earlier this season because he didn’t consider the Nets a championship contender, scored 21 points and grabbed nine rebounds in 16 minutes — helping the Heat to a 104-87 victory.

“It’s important that each and every one of us stay ready,” said Mourning, whose high in 19 games in his late-season return to the Heat was only 11 points. “And when an opportunity like this presents itself, you’ve got to take advantage.”

Dwyane Wade had 17 points and 10 assists for Miami. O’Neal, Eddie Jones and Damon Jones each finished with 14 points for the Heat, which moved to 38-5 at home this season.

“The Hulk carried Superman today,” O’Neal said.

Nenad Krstic had 27 points for New Jersey, which got 21 points from Vince Carter, 14 from Richard Jefferson and 10 from Jason Kidd.

“We’re still confident. We’re still focused,” Carter said. “And we still believe we can make this a contest.”

The series shifts Thursday to New Jersey, where the Nets have won 10 of their last 13 playoff games.

“They held serve,” Nets coach Lawrence Frank said. “Now, we’ve got to go back to Jersey and respond.”

Once a, the Nets found a way to hold O’Neal in check. And it didn’t matter.

In five games against the Nets this season — all Miami victories — O’Neal has been mortal, averaging 16.4 points and 10 rebounds. Still, the Nets still haven’t been close; their average margin of defeat is 19.8 points.

Mourning had 13 points in seven second-half minutes, eight of those points coming in the third quarter. His short hook with 3:46 left in the third gave Miami what was then its biggest lead, 69-49 — and capped a 16-5 run that put the Nets away.

“There must be something about the water down here that has energized him,” Nets forward Brian Scalabrine said. “He’s not getting tired.”

Said Heat coach Stan Van Gundy: “He was absolutely great tonight, maybe the biggest reason from a personnel standpoint that we win the game.”