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The year is only three weeks old, and the Buffalo Bills already have suffered a major loss.

Mike Pettine, the coordinator who oversaw a fiercely entertaining defense, is gone.

Pettine on Thursday signed a five-year contract to coach the Cleveland Browns.

At his introductory news conference, Pettine called it his “lifelong dream to be an NFL head coach.” He wore an orange tie. His lapel pin was a Browns helmet.

“Looking back on my year with the Bills, it’s a bittersweet thing for me to leave there,” Pettine said. “I had such a tremendous year there with that organization.”

Pettine said Bills coach Doug Marrone helped him prepare for the Browns interview. The Browns met with Pettine three times in eight days before making it official.

The Bills must regroup. They soon will name their fifth defensive coordinator in six seasons. Pettine also could take some of his assistants with him.

Pettine would like to hire Bills linebackers coach Jim O’Neil as his defensive coordinator. O’Neil also worked on Pettine’s staff with the New York Jets.

The Bills can deny Pettine’s wishes because O’Neil is under contract and might be a candidate to replace Pettine here. The Browns reportedly asked the Bills for permission to speak with O’Neil.

Other possible defectors could be defensive line coach Anthony Weaver and assistant secondary coach Samson Brown. They also came to the Bills with Pettine.

Bills General Manager Doug Whaley said at the Senior Bowl in Mobile, Ala., the front office has a list of coordinator candidates ready to go and would begin the search to replace Pettine immediately.

Whaley has declined to reveal the names of any candidates.

The Bills’ only assistant with previous NFL defensive coordinator experience is secondary coach Donnie Henderson. He ran the Detroit Lions’ defense in 2006 and the Jets’ in 2004 and 2005.

The most notable defensive minds looking for work are former Bills coach Wade Phillips, former 49ers coach Mike Singletary and two recently fired head coaches, Jim Schwartz of the Lions and Greg Schiano of the Tampa Bay Buccaneers.

Former St. Louis Rams coach Steve Spagnuolo is a special assistant for the Baltimore Ravens. Eric Mangini, previously head coach for the Browns and Jets, is a San Francisco 49ers consultant. Each could be interested in being a coordinator again.

Respected assistants possibly ready for promotions include 49ers defensive line coach Jim Tomsula, Seattle Seahawks passing-game coordinator Rocky Seto, Pittsburgh Steelers linebackers coach Keith Butler and Minnesota Vikings defensive line coach Brendan Daly.

Linebackers coach Pepper Johnson left the New England Patriots on Tuesday.

“We have a plan in place and we will introduce our new defensive coordinator after our final decision has been made,” Marrone said Thursday in a statement released by the Bills.

Many around the league are surprised Pettine has gotten the chance to be a head coach so quickly.

Just a year ago, Pettine struck out on his own. He was Rex Ryan’s sidekick for 11 years, breaking into the NFL with an entry-level job on the Ravens’ staff in 2002. Ryan was a Ravens assistant at the time. The two grew close.

When the Jets hired Ryan in 2009, he brought Pettine with him. Although Pettine was the Jets’ coordinator for four years, the defense was Ryan’s baby.

Browns fans certainly will view Pettine as an unproven commodity, something they’ve grown disgusted with over the years. The Browns fired Rob Chudzinski after one year. Before him, Pat Shurmur was a rookie coach who lasted two seasons.

Given the Browns’ dysfunction and the number of coaches who pulled out of their coaching derby, some observers wondered if remaining a Bills coordinator might be a more prudent long-term career move for Pettine.

“There’s only 32 of these jobs in the world,” Pettine said, “and these opportunities don’t come along often.

“People ask me, ‘Why didn’t you wait? There will be chances next year.’ I don’t know if I believe in that. Again, I looked at the situation as when you put all of the factors together this franchise is in position -- given the right leadership -- to win.”

Bills fans are disappointed to see Pettine leave. He’s the first Bills assistant to become another team’s head coach since the Indianapolis Colts hired Ted Marchibroda 22 years ago.

Buffalo was a wreck before Pettine arrived. Previous defensive coordinator Dave Wannstedt ran a stale, passive 4-3 scheme that got trampled on a weekly basis.

Pettine turned essentially the same personnel into a decorated unit that caused opponents trouble every week.

Four Bills -- pass-rusher Mario Williams, defensive tackles Kyle Williams and Marcell Dares, safety Jairus Byrd -- were selected for the Pro Bowl.

Inside linebacker Kiko Alonso, is a top candidate for defensive rookie of the year.

Pettine helped salvage a few careers, too. Players who’d looked like busts enjoyed remarkable seasons, namely pass-rusher Jerry Hughes, cornerback Leodis McKelvin and safety Aaron Williams.

“Congratulations to Mike Pettine for becoming Cleveland Browns new Head Coach,” Hughes tweeted. “Hate to see him leave.”

Aaron Williams tweeted a nearly identical sentiment, then followed up by noting he’s about to have his fourth defensive coordinator in four pro seasons.

“Hate to see Pettine go,” tweeted Bills captain and running back Fred Jackson, “but I’m excited for him to get an opportunity to be a head coach!”

The Bills ranked 10th in total defense, 28th in run defense and fourth in pass defense. They were gashed for long runs, allowing a league-high 19 rushes of 20-plus yards, but they hardly resembled themselves from the previous season.

The 2013 Bills were aggressive and dangerous. They set a club record and ranked second in the NFL with 57 sacks. They were second in interceptions.

“He put guys in position to make plays,” Bills safety Jairus Byrd told The

News from Honolulu, site of the Pro Bowl. “That’s what it was about on defense.”

The Bills allowed only seven defensive touchdowns against the Browns’ AFC North opponents, which included the defending Super Bowl champs, the eventual division champs and the Pittsburgh Steelers at Heinz Field. Ravens quarterback Joe Flacco threw a career-worst five interceptions against the Bills. The Bills should’ve beaten the playoff-bound Cincinnati Bengals, but lost in overtime.

“To compete in the AFC North,” Pettine said, “you have to be willing to bloody your nose a little bit. That’s the mentality we’re going to take here.

“This team’s going to be built on toughness. Most people think of toughness in just the physical sense. I think as important or more important is the mental toughness, the ability to think through things when they aren’t going well, to hang tough when things go bad, that the heads don’t drop, and it’s ‘Same old Browns,’ and teams talk themselves into losing.

“That, to me, is the culture that needs to be changed here.”

email: tgraham@buffnews.com.