When looking at possible replacements, there are three relevant categories: Coaches already on staff, coaches on other NFL teams with head coaching experience and assistants on other NFL teams looking for their first head coaching job.
Hiring a college head coach is always a possibility, but it’s becoming less common and would seem to be a longshot this season given the large amount of job swapping that has already taken place. Although Notre Dame coach Brian Kelly is a name worth watching.
›› How Bengals coaches have exited in the past
If the Bengals decide to promote from within this time, there are two viable candidates (linebackers coach Jim Haslett has head coach experience, but that ended 12 years ago and he’s only been in Cincinnati two seasons):
He is in his 15th season as an NFL coach and 13th with the Bengals, the last four of which have been as defensive coordinator. Guenther also worked as an offensive coach for the Washington Redskins earlier in his career.
His defense ranks 19th this season despite being ravaged by injuries. Guenther's unit ranked 17th in 2016, 11th in 2015 and 22nd in 2014.
›› PHOTO GALLERY: Marvin Lewis through the years
In charge of Bengals special teams the last 15 years, Simmons has been an NFL coach for 20 seasons, previously working in Baltimore and Carolina.
As special teams coordinator, Simmons has worked with almost every player on the roster at some point. There are a handful of head coaches with special teams backgrounds, but usually other promotions are involved beforehand.
It is worth noting that when Lewis was missed a practice during training camp due to a health issue, Simmons was the coach Mike Brown put in charge of the team for the day.
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When looking at the other 31 coaching staffs in the league, there currently are 17 coordinators with head coach experience. But there could be as many as eight current head coaches looking for work on or around Jan. 1, including a pair of former Bengals assistants.
He’s under contract as the head coach in Washington through 2020, but things have gone sour with the Redskins and owner Daniel Snyder has a history of firing coaches with time remaining on their deals.
Gruden, 50, was Bengals offensive coordinator from 2011-13 and a big reason why the team drafted Andy Dalton.
The former Bengals offensive coordinator has two years left on his contract in Cleveland, and Browns owner Jimmy Haslam said Jackson will be back in 2018 despite a 1-29 record.
But the Browns also brought in a new general manager in John Dorsey earlier this month, and if the Cleveland loses its last two games to finish 0-16, there could be a change and the original plan of the 52-year-old Jackson succeeding Lewis in 2018 could end up coming to fruition.
›› 6 things to know about Bengals loss to Vikings
Currently the offensive coordinator in New England, McDaniels was the head coach in Denver for two seasons, going 12-20. Eight years would seem to be a long gap between head coaching jobs, but the success of the New England offense is undeniable. The question is how much of it is a product of Daniels, and how much is due to Tom Brady?
The Ohio native has 17 years of NFL experience despite being only 41.
The Los Angeles Chargers offensive coordinator was the head coach in Arizona for six seasons, guiding the Cardinals to Super Bowl XLIII, and he spent two years as head coach in Tennessee.
His offensive in San Diego ranked 14th last year and is seventh this season. The Bengals are the only AFC North team the 55-year-old has not worked for in his 21-year NFL career.
›› Where did it go wrong with Marvin Lewis and the Bengals
The other current coordinators with NFL head coaching experience are: Terry Robiskie (Tennessee OC), Rob Chudzinski (Indianapolis OC), Marty Mornhinweg (Baltimore OC), Mike McCoy (Denver co-OC), Scott Linehan (Dallas OC), Pat Shurmur (Minnesota OC), Leslie Frazier (Buffalo DC), Dick LeBeau (Tennessee DC), Gregg Williams (Cleveland DC), Gus Bradley (San Diego DC), Jim Schwartz (Philadelphia DC), Dennis Allen (New Orleans DC), Mike Smith (Tampa Bay DC), Dom Capers (Green Bay DC), Wade Phillips (Rams DC).
As far as external candidates with no head coach experience, here are four – two offensive, two defensive – who many believe are ready to run their own teams.
DETROIT, MI - JANUARY 01: Offensive coordinator Jim Bob Cooter of the Detroit Lions watches his team against the Green Bay Packers during first half action at Ford Field on January 1, 2017 in Detroit, Michigan. (Photo by Leon Halip/Getty Images)
Jim Bob Cooter
The Detroit offensive coordinator for the last three seasons had a few head coach interviews last offseason and is regarded as one of the hot young candidates around the league. Just 33, Cooter is in his ninth season as an NFL assistant.
At 56 he’s a little older than most assistants looking for their first job as a head coach, but Reich has done a great job with Carson Wentz as the offensive coordinator in Philadelphia, where the Eagles are the No. 3-ranked offense in the league.
Reich is in his 10th season as an NFL coach. Prior to Philadelphia, he was the offensive coordinator with the Chargers for two seasons, raising the offense from ranking 18th to ninth.
The 42-year-old Ohio native who starred at Ohio State has a bright future, but he’s only in his third season as an NFL coach and first as defensive coordinator in Houston. But he has a winning pedigree with Super Bowl rings as a linebacker for New England, and he likely would come cheaper than some other candidates due to his lack of experience.
Like Vrabel, Wilks is in his first season as a defensive coordinator, but he has the Panthers ranked fifth in total defense after the group finished 19th last year. The 48-year-old Wilks has college head coaching experience at small Savannah State, rising from there to work at programs such as Notre Dame and Washington before becoming an NFL assistant in 2006.