Roger Clemens’ former strength coach saved medical waste from injections he said he gave the pitching legend in case “something did go wrong,” he testified Tuesday.

Brian McNamee said his relationship with the baseball star led to marital problems, and that his wife pressed him to do something to protect himself because he had a “history of taking hits for people and always getting in trouble.”

In the midst of what he called a “Battle Royale” with his wife, he said, she told him “you are going to go down,” explaining why he kept a needle, gauze and cotton balls from a 2001 steroid injection that he would later turn over to law enforcement officials.

View Photo Gallery: The trial of Roger Clemens.

McNamee is a key witness for federal prosecutors who are trying to prove that Clemens lied to Congress when he denied using steroids or human growth hormone in interviews with House investigators and lawmakers in 2008.

Clemens is charged with perjury, making false statements and obstruction of Congress.

McNamee described in vivid detail how, during a two-year period, he injected Clemens with performance-enhancing drugs near the jacuzzi in the New York Yankees clubhouse and at the pitcher’s Upper East Side apartment, where he recalled asking the doorman, Carlos, to keep an eye on his car so it wouldn’t get towed.

After arguing with his wife about the injections, McNamee testified that he tried to appease her by bring home a Miller Lite can stuffed with medical waste from Clemens. He initially stored the items in a FedEx box he marked “Clem” in a locked cedar closet and said he didn’t “plan on doing anything with it.”

McNamee, however, became a source for former senator George Mitchell’s 2007 report on performance-enhancing drugs in major league baseball. That report was the subject of a nationally televised Congressional hearing, which included testimony from McNamee and Clemens.

On the stand Tuesday, McNamee also recalled teaching Debbie Clemens, Roger’s wife, how to inject herself with HGH in the master bathroom of the couple’s home in 2003. McNamee said he felt “creepy” injecting his employer’s wife, and that he would not have done it unless Clemens was home.

“I can’t believe you’re going to let him do this,” McNamee recalled Debbie Clemens saying to her husband.

“He injects me. Why can’t he inject you?” Clemens responded, according to McNamee.

Clemens told Congress that he was not present when his wife was injected, and that he learned about it after it happened.

Defense attorneys are expected to try to skewer McNamee when they get a chance to question him — likely on Wednesday. In an effort to inoculate some of the potential damage, Assistant U.S. Attorney Dan Butler asked McNamee about his two drunk driving offenses within six months, his lies to police in a Florida criminal investigation and his own use of steroids and HGH.

U.S. District Judge Reggie B. Walton, who is presiding over the trial, began the day by explaining why he allows jurors to ask questions of witnesses, an uncommon practice among federal judges. Walton said he’s found that jurors stay more alert if they know they can ask questions.

Even so, Walton had to dismiss a second juror Tuesday for sleeping in the jury box.

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View Photo Gallery: A look at Clemens’s career.