Toronto Argonauts head coach Scott Milanovich has spent the last few weeks preparing a succession plan in case defensive co-ordinator Tim Burke decided he wouldn't return for the 2015 season.

On Friday, the CFL team made it official by announcing that Burke has resigned for personal reasons. He'll be replaced by Casey Creehan, who was linebackers coach last season and was originally tabbed to serve as defensive line coach this year.

"It's not the easiest of things," Milanovich said of the timing of the changes. "But when you have a plan and when you're prepared for this ... you're able to move pretty quickly and not miss a beat."

Milanovich did not reveal details surrounding Burke's departure, other than to say he did not believe it was health-related. He gave Burke his blessing to skip some off-season coaching meetings and said Creehan "basically acted as the co-ordinator" at those sessions.

"I sat in on all those meetings and watched him teach our new coaches ... and kind of give them the basis of our defence and what we're going to do," Milanovich said on a conference call. "He was extremely impressive, which I knew he would be. He did a great job. I know the timing sounds very late to you all but portions of this have been happening here for a little while.

"So Casey is not behind like you may think he is."

The moves come less than two weeks before the start of camp for quarterbacks and rookies. Toronto's pre-season opener is set for June 9 against the visiting Winnipeg Blue Bombers and the regular-season opener goes June 27 against the Edmonton Eskimos at Fort McMurray, Alta.

Burke joined the Argonauts in December 2013 as a replacement for Chris Jones, who had left the team to become the Eskimos' head coach. Burke first came to the CFL in 2005 as a defensive backs coach with the Calgary Stampeders and later worked with Milanovich in Montreal under head coach Marc Trestman.

They helped guide the Alouettes to Grey Cup titles in 2009 and 2010. Milanovich was an offensive co-ordinator and Burke was a defensive co-ordinator.

Milanovich said Burke gave him the news Thursday over the phone.

"Obviously I wish him nothing but the best," Milanovich said.

Creehan, meanwhile, has over 15 years of coaching experience at various levels. He was the Hamilton Tiger-Cats' defensive co-ordinator in 2012 and held the same position with the Blue Bombers in 2013 while serving as Winnipeg's linebackers coach.

"Casey is an extremely energetic real person that lives, sleeps and breathes football and that rubs off on the players," Milanovich said. "They know there's no fakeness in him and he's going to do everything he can to put them in a position to be successful."

There have been several changes to the Argonauts' coaching staff in the off-season. New staff members include Scott Downing (special teams co-ordinator), Jaime Elizondo (receivers coach), Jonathan Himebauch (offensive line coach/assistant head coach), Peter Kuharchek (inside linebackers coach), Mike Preston (assistant offensive line coach/assistant special teams coach) and D.J. Wingate (outside linebackers coach).

Marcus Brady returns as offensive co-ordinator and Anthony Ierullo is back as running backs coach. Milanovich said the team is planning to hire a defensive backs coach, with an announcement possible sometime next week.

Toronto missed the playoffs last season after finishing third in the East Division with an 8-10 record.