Here are five things to know about Allen:
He's no "stick in the mud": That was the term Allen used this summer while defending himself against the "Odd Couple" comparisons that so many people (myself included) were making with him and Ryan. As I wrote in a detailed profile on the two coaches, Allen was brought on board as a senior assistant this year to be both a calming influence and a detail-oriented teacher to help counterbalance Ryan's less-disciplined approach.
Even Payton referred to the two as "Felix and Oscar" at one point, using the order and disorder of their respective offices to illustrate his point. But Allen tried to debunk the notion that he's too buttoned-up. "I think you guys seem to portray me as kind of a stick in the mud," Allen cracked to the media. "I like to have fun, too."
That "Odd Couple" pairing clearly hasn't netted results on the field this year, with the Saints threatening to rank among the worst defenses in NFL history in some categories, including opponents' passer rating and total yards allowed. But players have praised Allen for his intelligence and teaching ability, as he has worked mostly with the secondary.
He helped the Saints win a Super Bowl: Allen, 43, was an original member of Payton's first staff in New Orleans in 2006. He started as an assistant defensive line coach before being promoted to secondary coach in 2008. Allen helped lead a standout secondary that included Jabari Greer, Darren Sharper, Roman Harper and postseason hero Tracy Porter during the championship 2009 season.
Allen got an even bigger break in 2011 when Payton's friend, John Fox, asked if Allen would make a good coordinator for him with the Denver Broncos.
He helped expand Tebow-mania: That 2011 Broncos season will always be remembered as the year that quarterback Tim Tebow kept leading Denver to remarkable, last-minute victories and an improbable playoff win. But Allen's defense played a huge role in keeping those games close.
Allen helped lift the Broncos' defense from 32nd in the NFL in 2010 to 20th in 2011. ESPN analyst Brian Dawkins, who was a Pro Bowl safety under Allen that year, said Allen required players to grasp the fundamentals first. Then, when they proved they could do that, he opened up a more aggressive playbook and more blitzes.
He flamed out in Oakland: Allen was hired as a head coach by the Oakland Raiders after just one year as a coordinator, but he was fired less than three years into the job after compiling an 8-28 record and starting 0-4 in 2014.
Reportedly, Allen's firing was simply about the lack of results he was getting on the field. The Raiders lost nine of those games by 20 or more points. It didn't help that they never had an established quarterback, with rising young star Derek Carr just beginning his career as Allen was let go.
Allen has said he believes having that perspective as a head coach has made him a better assistant.
He was devising third-down plan: ESPN NFL Insider Ed Werder reported that Allen was in charge of devising the Saints' third-down plan this year -- though Ryan was still making all of the calls on game day.
Third-down defense has actually been a rare strength for the Saints defense this year. They rank sixth in the NFL in opponents' third-down conversion rate (34.8 percent).
However, Allen's duties also include working heavily with a secondary that has been plagued by both deep passing plays and penalties. The Saints lead the NFL with 13 passing plays and seven TD passes allowed of 40-plus yards, and they lead the NFL with 42 defensive penalties.