The expert, who has been studying whether there were problems with government informants in the case, is expected to return in January.
In court papers filed Thursday, prosecutors agreed that the trial should not begin without the expert, Gregory Lee.
Prosecutors say that even if logistics could be worked out for him to testify, the Army has a regulation that bars active personnel from testifying as expert witnesses against the nation's interest.
They suggest moving ahead with jury selection as planned on Sept. 29, but not starting testimony until January.
U.S. District Judge Robert Kugler has scheduled a hearing on the issue for Sept. 4 in his Camden courtroom.
Even if the trial is held early in 2009, it would be a notably short time from the date of arrest to the start of such a complicated terror plot case.
The five men - foreign-born Muslims in their 20s who have lived much of their lives in southern New Jersey - were arrested in May 2007. The government said they were planning an attack on Fort Dix, a New Jersey installation used mostly to train reservists and National Guard members for active duty in Afghanistan and Iraq.
No attack ever took place.
The men are charged with conspiracy, attempted murder and various weapons offenses. They could all be sentenced to life in prison if they are convicted.
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