This might strike some fans as risky, when old hands such as Marty Schottenheimer and Bill Cowher might be available.
But hiring an "untested" assistant is less dicey than perceived and is, in fact, common practice in the NFL.
Many of the league's greatest coaches, from Vince Lombardi to Joe Gibbs to Bill Parcells, had never been head coaches when they began their first successful runs. They arrived with clear plans and total self-confidence, and benefited from supportive front offices.
Of the current coaches who were hired as first-timers after working as assistants, Andy Reid, Mike McCarthy, Jeff Fisher, Marvin Lewis, Sean Payton, Eric Mangini, Mike Tomlin and Lovie Smith have taken teams to the playoffs. Brad Childress, Romeo Crennel, Ken Whisenhunt, Gary Kubiak and Rod Marinelli have overseen improvements to previously stumbling franchises.
Those who've struggled include Cam Cameron, recently fired after a 1-15 season in Miami; Mike Nolan, who is 16-32 since he left the Ravens for San Francisco; and Scott Linehan, who couldn't overcome a rash of injuries in St. Louis this season.
The Ravens had a positive experience with a former assistant, hiring Brian Billick after he directed a record-setting Minnesota Vikings offense and winning the Super Bowl two years later.
Hiring anyone without head coaching experience might be considered a gamble, former New York Giants head coach and Ravens offensive coordinator Jim Fassel said.
"You're trying to project into doing one thing and now he's got to do another, but that's where most everybody at one time or another came from," Fassel said. "You've just got to evaluate that person. Can he make the transition? It's more of a gamble than a guy with a proven track record, but that doesn't eliminate you."
Garrett stands out not because he has no head coaching experience but because he has just three years of coaching experience, period. That's fewer than any other former assistant who is now a head coach in the league. Linehan had four years of NFL experience when the Rams hired him, though he had coached in college before that.
Of course, Garrett played quarterback in the league for 12 years, a tenure that only Kubiak can approach among current coaches.
Harbaugh is an unusual candidate because he has never been an offensive or defensive coordinator.
But his boss, Reid, had never been a coordinator when he was hired over several candidates with head coaching experience in Philadelphia. He has gone 88-56 and led the Eagles to the Super Bowl.
Throughout league history, the most successful coaches have come in with detailed plans and followed them. Some, such as Bill Walsh (who was a head coach at Stanford but made his reputation as an NFL assistant), were inspired technicians. Others, such as Parcells, were known for manipulating the moods of their teams. But all knew what they wanted and accepted no threat to their authority.
Lombardi, the most iconic of coaches, did not come in with a revolutionary system. Instead, he imposed repetitive drills until every man on the team did everything the way he wanted on every play. It was his conviction that carried the day.
"Everything was accounted for, labeled, identified, put in order, fundamental and sound," wrote Lombardi's biographer, David Maraniss. "You could tell that the coach believed in what he was doing. His tone of voice, his posture, his manner, it all made you believe."
Conviction seems to link great coaches, but it is no guarantee of instant success. Consider Bill Belichick's tenure in Cleveland.
It should have been the perfect marriage, wrote David Halberstam in The Education of a Coach: "The people of Cleveland were desperate to win; they had come close to going to the Super Bowl three times in the mid- and late 1980s, and here was this exceptionally thoughtful, careful, disciplined young man, about whom everyone spoke in such superlatives, arriving just in time to take them the final step. Nothing in his world seemed to take place by accident, and everything always seemed to be part of a larger plan."
But Belichick inherited an aging team and struggled to get along with owner Art Modell and the local media. His technical skills were exceptional, as he later demonstrated in New England, but he could not manage the larger situation.
"You can be right, but sometimes when you are right you are wrong, too, which is what happened in Cleveland," Halberstam concluded.
Belichick's failed first attempt illustrates many of the pitfalls that swallow one-time assistants.
When Chris Palmer took the Cleveland job in 1999, he received an expansion team with little talent. He learned that he had philosophical differences with the franchise's management and quickly lost the respect of some players. He was fired after going 5-27 in two seasons.
Marty Mornhinweg took over the Detroit Lions in 2001 with a reputation as an excellent offensive mind. But he inherited a mediocre quarterback in Charlie Batch and an underachieving offensive line. He set a poor tone for his reign when, disgusted with his players, he called off a summer practice and drove away on his motorcycle.
He also struggled to charm the Detroit media and lost credibility by waffling on his support for Batch. He was also fired after two seasons with a 5-27 record.
Dick LeBeau was among the league's most respected defensive coordinators when he became the Cincinnati Bengals' head coach in 2000. But he inherited an uncertain quarterback situation, an untalented defense and a general manager with a poor track record in Mike Brown.
LeBeau went 12-33 in three seasons and was fired. He returned to leading Cowher's defense in Pittsburgh and helped the Steelers to a Super Bowl win.
In few of these cases did coaches fail because of a lack of technical proficiency. They were buried by thin talent, an inability to gain respect from players, poor public relations skills and/or failure to get along with their bosses.
Of the current crop of assistants turned head coach, more were offensive coaches than defensive coaches. But offensive coaches haven't been notably more successful than their defensive peers.
As Billick showed in Baltimore and Childress has demonstrated in Minnesota, a reputation as a great passing mind does not necessarily translate to instant offensive improvement.
Quality players remain the NFL's prime currency.
Coaches who oversaw drastic offensive leaps, such as Reid and Payton, often benefited from significant roster upgrades. In Philadelphia, Reid got to work with a maturing Donovan McNabb, while his predecessor had been stuck with Rodney Peete and Ty Detmer. In New Orleans, Payton got Drew Brees rather than Aaron Brooks.
Sun reporter Don Markus contributed to this article.
A list of great NFL coaches who were hired despite not having head coaching experience:
Hired by: Green Bay Packers in 1959 at age 45.
Previous experience: Offensive coordinator for New York Giants.
Record: Went 89-29-4 with Packers, won five NFL championships and first two Super Bowls.
Hired by: Washington Redskins in 1981 at age 41.
Previous experience: Offensive coordinator for Tampa Bay Buccaneers and San Diego Chargers.
Record: Went 124-60 with three Super Bowl titles in first tenure with Redskins.
Hired by: Giants after 1982 season at age 41.
Previous experience: Defensive coordinator for Giants.
Record: Went 77-49-1 and won two Super Bowls with Giants.
Hired by: Oakland Raiders in 1969 at age 32.
Previous experience: Linebackers coach for Raiders.
Record: Went 103-32-7 for modern-NFL-record .763 winning percentage and won a Super Bowl.
Hired by: Pittsburgh Steelers in 1969 at age 37.
Previous experience: Defensive coordinator of Baltimore Colts.
Record: Went 193-148-1 and won four Super Bowls with Steelers.
Hired by: Colts in 1963 at age 33.
Previous experience: Defensive coordinator for Detroit Lions.
Record: Went 71-23-4 with Colts and won an NFL championship.