The defending NBA champions lost at home for the third game in a row Friday night, when the New York Knicks took advantage of O'Neal's absence to win 100-76.
The Heat announced shortly before tipoff that O'Neal, sidelined since Sunday, will undergo surgery for torn knee cartilage and be out four to six weeks. It's a familiar pattern for the Heat: Last season, O'Neal sprained his ankle in the home opener and missed the next 18 games.
"Obviously we're going to have to gather the troops and do something different," coach Pat Riley said. "I think the guys have sort of been waiting for him to come back. Now we have to look in another direction."
O'Neal watched from the bench as Knicks center Eddy Curry made all five of his shots in the third quarter, when New York shot 78 percent and outscored Miami 34-14.
"I wish Shaq well," Curry said. "But I didn't miss him tonight. I know I dodged a bullet."
Reserve Jamal Crawford scored 19 points and Steve Francis added 14 for the Knicks, who have won their past two games by a total of 44 points. New York got 46 points from its bench and outshot the Heat 52 percent to 36.
"This has been the most balanced victory so far," Crawford said. "It feels good to come in and spread the ball out and make plays."
Miami fell to 3-5 and lost to New York for the first time in their past eight meetings. The Heat have lost three times by 22 points or more, each time at home.
"I can't explain this at all," Riley said. "I can't remember this in the 11 years I've been here -- getting beat so badly at home with a good team. We're not defending. We're not playing hard enough. We're surely not scoring. Right now it doesn't look good at all."
The Heat went 10-13 when O'Neal sat out last season before their postseason run.
"We were without Shaq last year and didn't play like this," backup center Michael Doleac said. "Something's off. It starts in our heads."
There was more bad news for Miami: Jason Williams, a starter last season, sounded discouraged following his first game since offseason arthroscopic knee surgery.
He had eight points and four turnovers in 18 minutes.
"My knee felt OK in the beginning, then got a little tight in the second half," Williams said. "I just don't feel like me."
Dwyane Wade led the Heat with 20 points. Alonzo Mourning, starting for O'Neal, managed only seven points and seven rebounds in 24 minutes. Antoine Walker, Dorell Wright and James Posey shot a combined 2-for-21.
The Knicks led for most of the first half, then scored the first seven points of the third quarter to go up 61-49. Miami missed eight consecutive shots, and New York ran off 12 points in a row to lead 78-55, prompting scattered boos.
The jeers were louder at the end of the third period, when the Heat trailed 88-63.
"This is how you build character," Wade said. "We're down now, but we know the true test of a champion is how they fight back."
Williams echoed that sentiment.
"It's not like they're going to cancel any games for us," he said. "We've got to keep going."
Despite their recent struggles against Miami, the Knicks lead the series 47-30. ... The Heat play eight of their next 11 games on the road, beginning Saturday at Atlanta. ... Miami trailed after one quarter for the sixth time in eight games.