The Nets had scheduled a news conference for Thursday morning to introduce their prized off-season acquisition, power forward Shareef Abdur-Rahim. Instead, Rod Thorn, the Nets' president, announced that the trade for Abdur-Rahim was in limbo because of an unresolved matter in connection with his physical on Wednesday.

Thorn would not specify why the 6-foot-9 Abdur-Rahim did not pass the physical, but he said that the problem did not involve a pre-existing condition.

Abdur-Rahim, a nine-year veteran who spent the past season and a half with Portland and had become a free agent, had signed a contract with the Nets, but it will not become official until the Nets are satisfied with his physical condition. The six-year deal to bring him to New Jersey, worth close to $39 million, was part of a sign-and-trade deal with the Trail Blazers.

The Nets are seeking other medical opinions. If they do not complete the trade by Aug. 12, it will fall through.

"It was a shock," Thorn said of the development. "We had scheduled a press conference. Obviously, we did not think there was going to be anything wrong whatsoever."

Abdur-Rahim, who is in Atlanta, was also stunned, Thorn said.

Abdur-Rahim had surgery on his right elbow last season and missed 28 games.

"He had a knee operation when he was in high school," Thorn said. "He's never missed a game with the knee problem in the N.B.A.; never missed a practice or game. The only thing he's ever missed any games for in the N.B.A. was for an elbow operation."

When asked if the condition was career-threatening, Thorn said, "That's not for me to say, one way or the other."

If the deal falls through, it would be an enormous blow to the Nets. Abdur-Rahim, 28, was considered the best power forward on the free-agent market. He averaged 16.8 points and 7.3 rebounds last season and was an All-Star in 2002 with Atlanta.

The Nets were hoping to play him alongside Jason Kidd, Vince Carter, Richard Jefferson and Nenad Krstic. All five players averaged 10 or more points last season.

"He was a big piece to our puzzle and a quality player," Thorn said. "So if it works out that we don't complete the trade, then we've lost a quality player."

Thorn said he remained hopeful that Abdur-Rahim would wear a Nets uniform this season, but that a contingency plan was already in motion.

"We've started looking at other options because players that we may have an interest in are either signing every day or they are about to sign," Thorn said.