(adds Quinn quotes)
By Martyn Herman
LONDON, Dec 4 (Reuters) - Roy Keane quit as Sunderland manager on Thursday just five days after a 4-1 home defeat against Bolton Wanderers prompted the Irishman to question his future at the Premier League club.
In a statement on its website the club said an amicable agreement had been reached with the 37-year-old Keane over his departure, two and a quarter years after taking charge.
Sunderland have slipped to 18th position in the Premier League after a slump in form.
Chairman Niall Quinn, who was instrumental in bringing Keane to the club, said he was disappointed that Keane had decided to leave and said he had spent three days trying to get him to change his mind.
"I've got huge respect for what he has done for this club, he lifted it off its knees," Quinn told reporters at a news conference at the Stadium of Light.
"I fully respect his decision. He is his harshest critic and he felt like he had completed his journey here and that maybe somebody could come in and kick it on.
"I know Roy and I knew it was too late to make him change his mind. I think we all know that the Premier League has not seen the last of him."
Former Manchester United and Ireland midfielder Keane took over in August 2006 after Sunderland, relegated the season before, had made a terrible start to the season under the temporary charge of Quinn.
His first stab at coaching proved an instant success as he steered the north east club to the Championship (second division) title and back into the Premier League.
Sunderland finished 15th in the Premier League last season and were expected to improve on that after some heavy spending in the transfer market.
However, five defeats in their last six league games have left them in the relegation zone, although they are only four points from mid-table security.
Saturday's defeat at home to Bolton in Keane's 100th match in charge was the club's fifth defeat in eight home league games this season and appears to have been the final straw for Keane.
"I would like to thank my staff, players, Niall Quinn and in particular the fans for their support during my time at Sunderland and I would like to wish the club every success in the future," Keane told the club's website.
Sunderland said Ricky Sbragia would take charge of the team for this weekend's match against Manchester United at Old Trafford with assistance from Neil Bailey and Dwight Yorke (Editing by Miles Evans)comments