Belichick's expertise has caused many to overlook Crennel's role in New England's success because there is still a label that this is Belichick's defense. The Patriots argue otherwise.
"I know Bill gets a lot of credit, but Romeo is just as important as Bill is," linebacker Tedy Bruschi said. "I have been there on the sideline where Bill is on my left and Romeo is on my right and they make the adjustment in front of me.
"They'll be going at it, batting things back and forth. Sometimes, Bill prevails. Sometimes, Romeo does. I know no matter which one gets his wish, I know that play, that call, that adjustment is going to work."
What can't be ignored is Crennel's resume.
He has been coaching for 35 years, the past 24 in the NFL. He has been on the coaching staff of four Super Bowl champions.
"I think I'm ready to be a head coach," Crennel said. "I started off in special teams and was successful there. I was successful as a defensive line coach and as a coordinator. I think you'll see that I can handle myself and handle players and be able to handle a team.
"I'm going to do the job I've always done. Be analytical, be hard-working, be efficient and then we'll see what happens."
Crennel's defenses have been consistently stingy, leading the NFL in fewest points scored in 2003 (14.9) and finishing second in 2004 (16.2).
His unorthodox schemes keep offenses off-balance. The Patriots can shift from a 3-4 defense to a 4-3 to an alignment with no down linemen at all (using six linebackers instead).
Crennel might have been at his best in this year's playoff run. Without both starting cornerbacks (Ty Law and Tyrone Poole) and All-Pro defensive end Richard Seymour, New England forced seven turnovers while giving up three touchdowns in two postseason games.
"He understands the talent he has and always puts people in the best position to get the job done," linebacker Willie McGinest said.