"I think he just pushed guys too far," Boone told HBO's "Real Sports with Bryant Gumbel" for an episode that will air Tuesday. "He wanted too much, demanded too much, expected too much. You know, 'We gotta go out and do this. We gotta go out and do this. We gotta go out and do this.' And you'd be like, 'This guy might be clinically insane. He's crazy.'"
Said Harbaugh, when asked by HBO whether he has a problem working with others: "It must be true. Because I'd wear out my welcome. People just don't want to be around you for a while."
In September, when reports were swirling about Harbaugh's future with the team, Boone was one of his more staunch, vocal defenders. Harbaugh was introduced as coach of the Michigan Wolverines -- his alma mater -- on Dec. 30.
"I know for a fact that everybody loves Harbaugh," Boone told 95.7 The Game in September. "He's a great guy. How can you not want to win for a guy that wears cleats during the game? Come on now. Have you not seen that guy's energy? He's excited 24-7. You've got to love to play for a guy like that. That's what football is all about."
But now? Boone, who played for Big Ten rival Ohio State, has flipped on his former coach.
"He does a great job of giving you that spark, that initial boom," Boone told HBO. "But after a while, you just want to kick his ass. ... He just keeps pushing you, and you're like, 'Dude, we got over the mountain. Stop. Let go.' He kind of wore out his welcome."
It's not that Boone doesn't think Harbaugh will be successful at Michigan. That's not the case at all, as Boone told HBO he has no doubt that Harbaugh is at the right place.
"I think he's a better college coach than he would be as an NFL coach," Boone told HBO. "It's different for college kids than pro players. I don't think college kids can say, 'Hey, Jim, you're pissing me off today.'"
The 51-year-old Harbaugh played quarterback for four teams during a 14-year NFL career before retiring after the 2000 season to become a coach. He got his first head-coaching job at the University of San Diego in 2004, left for Stanford in 2007 and then jumped to the NFL with the 49ers in 2011.