By Ian Simpson

WASHINGTON (Reuters) - A key witness in the federal perjury trial of Roger Clemens, his former trainer, could testify on Tuesday, a prosecutor said on Thursday, setting the stage for a possible courtroom confrontation between the baseball legend and his ex-assistant.

Testimony from the witness, Brian McNamee, is critical to the government's charges that Clemens lied to Congress about using anabolic steroids and human growth hormone.

"I don't think we will reach Mr. McNamee until Tuesday," U.S. Assistant Attorney Steven Durham told Judge Reggie Walton as part of a discussion about scheduling.

Durham is expected to use McNamee's testimony to show that the trainer injected Clemens with anabolic steroids in 1998, 2000 and 2001 and with human growth hormone in 2001. Clemens' attorneys have called McNamee a liar.

Testimony was given on Thursday by former Internal Revenue Service agent Jeff Novitsky, who had investigated McNamee as part of a probe into performance-enhancing drugs.

The evidence he obtained included needles, syringes, swabs, vials and other materials turned over by McNamee to federal agents in January 2008.

In a potential blow on Wednesday to the prosecution's case, Clemens' former New York Yankees and Houston Astros teammate Andy Pettitte said he might have misunderstood Clemens when the ace pitcher told him in 1999 or 2000 he had used human growth hormone to recover from an injury.

The admission prompted lawyers for Clemens to ask that Pettitte's testimony about the critical conversation be struck from the record, since the pitcher cannot be sure of what he heard.

If Pettitte's testimony is removed, it increases the importance of McNamee's testimony to the prosecution.

Clemens is being tried for a second time on federal charges of lying to the House of Representatives' Committee on Oversight and Government Reform in 2008 about whether he used performance-enhancing drugs. The panel was investigating drug use in Major League Baseball.

His first trial ended in a mistrial last year.

Clemens, a seven-time winner of the Cy Young award, baseball's highest annual honor for a pitcher, is among the game's biggest names linked to alleged drug use.

The trial has recessed until Monday.

(Editing by Mohammad Zargham and Philip Barbara)