During games, he barks instructions from the bench with the delicacy of an 18-wheeler grinding its way uphill. With reporters, he's unfailingly polite but seldom makes eye contact. He comes across as shy and earnest, preferring to cede the stage to his players. His boss, Chicago Bulls general manager Gar Forman, says he likes to come in early and leave late.  "He told me he has a rare stamp collection," Forman said.  Tom Thibodeau during a game against the Hawks on May 8, 2011 in Atlanta.  Tom Thibodeau, a rumpled, 53-year-old bachelor, is the sort of guy who might look at home selling vacuum cleaners or manning a backhoe. Instead, he guided the Chicago Bulls to the NBA's best record this season and claimed the league's Coach Of the Year award. The Bulls are tied with the Atlanta Hawks at two games apiece in their best-of-seven Eastern Conference semifinal series. Game 5 is Tuesday at Chicago's United Center.  Within the celebrity culture of the NBA, where most coaches are former stars who wear pocket squares, Thibodeau is the guy with the precariously knotted tie whose basketball career maxed out in 1981 when he captained the Division III Salem (Mass.) State Vikings. Before the Bulls bucked convention and hired him last June, he'd worked his way up from coaching at his alma mater to joining the staff at Harvard and spending more than two decades as a journeyman assistant with six NBA teams. In 2008, he won his first NBA title as an assistant to Doc Rivers on the Boston Celtics.  "All coaches are a sum of their past experiences," Thibodeau said at a news conference this month. A team spokesman said Thibodeau was unavailable for an interview Monday.  During his 21-year slog to the top, Thibodeau established himself as a tireless worker and as something of a defensive savant. This season, the Bulls led the league in the categories of field-goal-percentage defense and rebounding margin—a fact that didn't come as a surprise to his friends and advocates.  "He was ready for this sort of opportunity 15 years ago," said Jeff Van Gundy, the former head coach of the New York Knicks and Houston Rockets. "I'll say this: Chicago wasn't worried about winning the press conference. They were worried about winning more games."  Former players have always had an advantage when it comes to nabbing head-coaching jobs. Since the 1980-81 season, 45% of the league's coaches played in the NBA or now-defunct American Basketball Association, according to Stats Inc.  The requirement seems to be getting firmer. This past season, 25 of the league's 33 total head coaches (including coaching changes) were former players. In Chicago, Thibodeau succeeded Vinny Del Negro, a former player who coached the Bulls to back-to-back .500 seasons and a pair of opening-round exits from the playoffs.  John Lucas, the former coach of the San Antonio Spurs, Philadelphia 76ers and Cleveland Cavaliers, said there's a perception in the NBA that ex-players are better able to relate to multi-millionaire athletes and command their respect. Others say that hiring someone whom fans recognize can give teams a bump in ticket sales and general public relations.  When the Bulls brought Thibodeau on board, nobody rioted in celebration. One columnist for the Chicago Sun-Times described Thibodeau as a "guy" who'd spent years "sleeping on ballbags."  There's no evidence that former NBA players actually make better coaches. Since the league's inception, they've combined for a .499 regular-season winning percentage. Coaches who never played in the NBA have a .501 all-time winning percentage. "There's no formula," Lucas said.  Thibodeau's work ethic has become the stuff of legend in basketball circles. John Galaris, a longtime friend and the former athletics director at Salem State, recalled visiting Thibodeau when he was an assistant with the Spurs. Thibodeau was up at 4 a.m. breaking down film. "I swear the guy only sleeps four hours a night," Galaris said.  Jon Barry, who played for the Rockets when Thibodeau was an assistant there under Van Gundy, said Thibodeau was an effective one-on-one teacher, citing the countless hours he spent working with Yao Ming on his footwork and the daily handwritten notes he made for each of the team's players.  Thibodeau certainly has benefited from the emergence of Bulls point guard Derrick Rose, the league's Most Valuable Player. But he has also achieved what has become the litmus test for modern NBA coaches: getting the team to listen. "Players are going to respect you if they think you know what you're doing on a daily basis," Forman said. "They know this is a guy who's prepared and has a plan."  The only danger with Thibodeau's success, Van Gundy said, is that the stories people tell sometimes portray him as some sort of drone who has prospered only by the volume of effort, rather than the quality. "He's not a beaver building a dam," Van Gundy said. "He's not just a guy who works, works, works. He also has a brilliant basketball mind."  Peter Roby, the athletics director at Northeastern University and the former head coach at Harvard, where Thibodeau spent four seasons as an assistant, said he hopes Thibodeau's success will give teams the "social permission" to consider more unconventional hires.  "He wasn't a former player, and he didn't have a big college reputation," Roby said. "He just happened to be someone who understood the NBA, could relate to players, was willing to outwork everyone and knew what he was talking about."  Write to Scott Cacciola at scott.cacciola@wsj.com
Tom Thibodeau of the Bulls is the rare NBA coach who's never played in the league. But his players still listen, and the team is succeeding.