Herzog, speaking at a news conference in St. Louis after being elected to the Hall of Fame on Monday, said he wants the move to work out for McGwire and the Cardinals but that it will require honesty from the former slugger and patience from manager Tony La Russa.
"I really want it to work out for the Cardinals, but I don't know," Herzog told reporters. "And we won't until we see how Mark reacts to all of this. Sometimes I say, 'Maybe he's still not going to do it,' maybe he's going to wake up one morning and say, 'I don't want to go through it.'"
McGwire was hired by the Cardinals to be the club's hitting instructor in late October when La Russa agreed to a one-year contract extension, but the Cardinals have yet to hold a press conference announcing the decision.
But if the team does hold a press conference, McGwire needs to be ready to answer questions about his past, according to Herzog.
"He's going to be asked questions about steroids, he's going to be asked so many things, and he's got to be open and he's got to answer," Herzog said. "And Tony can't get mad about it. He's got to put up with it."
McGwire was an elite home run hitter in his day as he mashed 583 career home runs with the A's and Cardinals over 16 Major League seasons. He also hit 70 home runs in 1998 to break Roger Maris' storied single-season home run record of 61.
But that same year, McGwire admitted he took androstenedione, an over-the-counter muscle enhancement product that was legal in the United States and in Major League Baseball at the time.
And McGwire faced further scrutiny in 2005, when he declined to answer questions under oath before the House Government Reform Committee about using performance-enhancing drugs, saying he "wasn't here to talk about the past."
McGwire has generally avoided talking to the media since that testimony, but as Herzog pointed out, there will be no place to hide if he takes the hitting coach job with the Cardinals.
"It's going to be tough because Mark has to open up and he has to be real open with the press," Herzog said. "If he doesn't, it's not going to be the fans and you guys in St. Louis as much as it's going to be going to Cincinnati, going to Pittsburgh, going to Philadelphia, going to New York."
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.