Prosecutors hope that Segui and Anthony Corso, who was one of McNamee's private workout clients in New York City, can help rebut defense suggestions that McNamee fabricated allegations and evidence against Clemens to avoid prosecution after being contacted by federal authorities in 2007.
The government wants to question Segui about a conversation from around 2001 in which McNamee is said to have told Segui he had saved ''darts'' - that is, needles - from his injections of players in order to placate his wife. That would be consistent with McNamee's testimony in the trial that he saved medical waste from the injections because his wife was worried he would end up getting the blame for being involved with drugs in baseball.
Corso is expected to testify that McNamee told him as early as 2002 that Clemens used HGH, and in 2005 that McNamee saved evidence from a 2001 injection.
McNamee, Clemens' former strength and conditioning coach, testified that he injected Clemens with steroids in 1998, 2000 and 2001 and with HGH in 2000. The former pitcher, a seven-time Cy Young Award winner, is accused of lying to Congress in 2008 when he said he never used performance-enhancing drugs.
Defense lawyers, who objected to the additional witnesses' testimony, argued Wednesday that they never contended that McNamee made up the evidence recently, but that it had been created and saved as a ''hole card'' - as a way to possibly extort Clemens one day.
But U.S. District Judge Reggie Walton said Thursday that the thrust of the defense cross-examination of McNamee was that he was motivated to fabricate the evidence after he came under investigation by federal authorities in 2007.
On Wednesday, prosecutors indicated they had encountered some resistance from Segui, who retired in 2004 after 15 major league seasons. That prompted Walton to warn that if Segui defies a prosecution subpoena to testify in the trial: ''He better be on the run, because the marshals will be after him.''
''If he doesn't show up, he'll be arrested like anyone else,'' Walton said calmly.
McNamee testified last week that after he injected Clemens with steroids in 2001 he stored the medical waste in a beer can to allay his wife's fears that he would become the fall guy if his involvement with drugs in baseball was ever exposed.
Hardin's zealous cross-examination of McNamee last week has given the government opportunities to seek admission of additional evidence. On Monday, Walton ruled that Hardin had opened the door for McNamee to name other players to whom he had supplied HGH, something the defense had fought vigorously to keep from the jury.
Also Thursday, a forensic examiner with the FBI said that tests on some of the vials, syringes and other items that McNamee handed over to authorities revealed residues of steroids on them. Under cross-examination, the examiner, Pamela Reynolds, said that she could not say how the steroids got there and who, if anyone, used them. The government will make that connection later, when it is expected to show that the medical waste contains Clemens' DNA.