With Pettine hired on Thursday, Marrone is forced to enact the contingency plan the team began preparing once Cleveland honed in on his key assistant.
''We have a plan in place,'' Marrone said, in a statement released by the team. ''I want to take this opportunity to congratulate Mike on becoming the Browns new head coach and thank him for all of his time and efforts with our team this past season. He did an excellent job of improving our defense, and we wish him the best.''
Marrone did not reveal a timetable or list of potential candidates.
Depending on whether any Bills assistants follow Pettine to Cleveland, Buffalo has several in-house assistants to consider, most notably linebackers coach Jim O'Neil and defensive backs coach Donnie Henderson.
O'Neil has spent the past five seasons working under Pettine, including the first four with the New York Jets. Henderson is the most experienced, having split 29 years as a coach at the NFL and college levels, including stints as a defensive coordinator with the Detroit Lions in 2006 and Jets, 2004-05.
Replacing Pettine is not something Marrone was anticipating a year after luring the former Jets defensive coordinator to Buffalo to become the most-high profile member of his first staff.
Pettine's departure is a significant blow to a defense that emerged as the Bills strength last season. And the loss has the potential of under-cutting much of the progress the Bills hoped would carry over into next season after a 6-10 finish that extended the NFL's longest active playoff drought to 14 seasons.
Continuity also becomes an issue for a defense set to be led by a fourth coordinator in as many years. Pettine succeeded Dave Wannstedt, who was fired along with Chan Gailey and his staff a year ago. Wannstedt replaced George Edwards, who struggled during his two years as coordinator.
Pettine, in one season in Buffalo, played a considerable role in transforming the Bills once-porous defense into an aggressive unit that finished second in the NFL with a franchise-record 57 sacks. Though the Bills were inconsistent against the run, they still finished 10th in the NFL in fewest yards allowed - Buffalo's best ranking since finishing second in 2004.
The Bills success on defense translated into four players earning Pro Bowl selections, including defensive tackle Marcell Dareus as a last-minute injury replacement. Also selected were defensive linemen Mario Williams and Kyle Williams and safety Jairus Byrd.
In November, Pettine indicated he was in no hurry to leave Buffalo. He enjoyed the community and also wanted to continue what he began building with the Bills.
Pettine's approach and no-nonsense demeanor was popular among players and staff. During the Bills bye weekend off in November, Pettine and a number of assistants spent a weekend sharing a cabin in nearby Ellicottville.
He also earned raves among several veterans, including Mario Williams, who led the team with 13 1/2 sacks.
Pettine learned his defensive approach under Jets coach Rex Ryan. And it's a style that's somewhat unique among NFL teams in placing a premium on players' versatility and capability of playing several positions to confuse offenses.
The secondary benefited in finishing second in the NFL with 23 interceptions.
The change at coordinator could have an effect on Byrd's desire to return to Buffalo. Byrd played last season under a one-year contract after the Bills retained his rights by applying the franchise tag. The Bills have the option of tagging Byrd once more this offseason, but are also making a push to sign the player to a long-term contract.
Despite initial reservations, Byrd enjoyed playing in Pettine's system and working with Henderson.
The defense also featured the Bills most promising youngster, linebacker Kiko Alonso, an NFL defensive rookie of the year candidate.
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