Former teammates of Roger Clemens may not be allowed to testify in the way federal prosecutors hoped would bolster the credibility of their star witness in their case against the legandary pitcher, a federal judge said Tuesday.

Prosecutors hoped the teammates would be permitted to testify that they knew they were being injected with performance-enhancing drugs by Brian McNamee, a former trainer and Clemens’ chief accuser.

McNamee is expected to testify that he injected Clemens with steroids and Human Growth Hormone (HGH) numerous times between 1998 and 2001 and that the former star pitcher knew what he was getting. Clemens has said he thought he was being given vitamins.

McNamee’s credibility is expected to be a major battleground during the lengthy trial, which starts Wednesday with jury selection.

Clemens, 48, the most decorated pitcher in baseball history, is accused of committing perjury and related offenses in 2008 testimony before a House committee during which he denied taking steroids or HGH during his playing days.

McNamee testified emphatically at the same hearing that he “injected those drugs into the body of Roger Clemens at his direction.”

Prosecutors hope to have former teammates of Clemens — Andy Pettitte, Chuck Knoblauch and Mike Stanton — testify that they were knowingly injected with performance-enhancing drugs by McNamee. Such testimony, prosecutors believe, would buttress McNamee’s version of events in respect to Clemens.

But U.S. District Judge Reggie B. Walton said during a brief hearing Tuesday that he felt the testimony of the other players may be unfair to Clemens.

“There is a real danger,” Walton said, “that the jury may say, ‘If they all knew, why wouldn’t Mr. Clemens know’” that he was being injected with the same kinds of substances.

Clemens, suntanned and wearing a dark suit with a light blue shirt and blue tie, sat quietly and took notes during the hearing. Surrounded by his legal team, he left the courthouse without taking questions from reporters.

At the same hearing, Walton indicated he would prevent defense lawyers from telling jurors that McNamee had been investigated in connection with the rape of a woman in 2001. (He was not charged with a crime in the case.)

However, the judge said it would be fair for defense lawyers to say that McNamee made false statements during a criminal investigation in 2001 without going into details of the underlying accusations.

Walton also worried that the trial might experience a costly delay because prosecutors have not been able to obtain a tape-recording of a deposition Clemens gave House investigators in 2008. They have obtained a transcript of that interview, but Assistant U.S. Attorney Steven Durham told Walton that the House has refused to turn over the tape because it is proprietary to the House reporter who transcribed the interview.

Defense lawyers are likely to challenge the admissibility of a transcript if an audio-tape of the same interview exists but is not being handed over to the court.