He'll be supervising the man he's replacing in that role - and try to help the Vikings revive a group that was the easiest in the league to throw a touchdown pass against this season.
Yes, this is a unique arrangement and a challenging job. For Williams, though, what's most important is his strong relationship with the guy who hired him, head coach Leslie Frazier. They shared responsible of the Colts defensive backs for two years, including the Super Bowl-winning team in 2006-07.
"It was another opportunity to work together, first of all, and to win some more championships," Williams said Thursday, after signing a two-year contract.
To get to that level, Frazier, Williams and the rest of the Vikings have a lot of work to do coming off a 3-13 season, beginning with a depleted secondary that gave up a NFL-most 34 touchdown passes against only eight interceptions in 2011.
Those struggles were contributing factors in Frazier's decision - he also said he sought some "new energy" - to dismiss defensive line coach Karl Dunbar and demote defensive coordinator Fred Pagac to linebackers coach.
"Any time you get a chance to come in with a lot of the building blocks already in place, you want to jump at that," Williams said on a conference call.
"We were fortunate that Alan was available," Frazier said, adding: "I was a little nervous that we might not be able to get the guy I thought would be able to help us."
Frazier will have a big hand in that help. He said he wasn't involved as he should have been in the defense at the beginning of the season, and he'll have heavy input with Williams, who has been schooled in the same 4-3 scheme and Tampa Two zone coverage the Vikings have used versions of for years.
"Having a full offseason together, I've got to believe is going to be a big plus for us," Frazier said.
But Pagac is staying, as is Frazier's old teammate, Mike Singletary, who remains an assistant head coach - and a linebackers coach. Neither Frazier nor Williams said they were sure yet how Pagac's and Singletary's duties would be divided.
Frazier said he wasn't concerned about a too-many-cooks-in-the-kitchen problem with running the defense.
"Hearing their feedback, that assured me that it could work," Frazier said. "If you don't have the right kind of people it can be a dicey situation."
Williams said he wasn't worried about Frazier micromanaging his job. He also said he wasn't concerned about the potential awkwardness of working with Pagac, the man he was hired to replace. If Frazier said this would be fine, well, that was all he needed to hear.
"I know his word is golden. I know his judgment is second to none," Williams said.
Williams said he doesn't believe "wholesale changes" are necessary for the secondary and vowed that the Vikings will be better and faster as a defense. While his personality is more muted than Tomlin's, he said they have a similar philosophy about how to teach their players and how to call a game.
"We're going to run. We're going to tackle. We're going to hit. We're going to play tough football better than our opponents," Williams said.
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