Crawley Town manager John Gregory is stepping down from his position with the League One club to undergo major open heart surgery.
Ex-Wolves and Doncaster manager Dean Saunders has been named interim boss.
Crawley said in a statement on their website that Gregory, 60, would be having an operation at the start of January before a lengthy period of recuperation.
The former Aston Villa boss had been in charge since December 2013.
Crawley said that he needed to undergo surgery as soon as possible but that his condition is not stress-related.
Gregory, who has also managed QPR, Derby County and Wycombe Wanderers, won six caps for England as a midfielder.
Crawley are 21st in League One and have not won in their last 10 games.
Former Wales, Liverpool, Aston Villa and Derby striker Saunders, 50, worked on the coaching staff at Blackburn Rovers and Newcastle United before starting his managerial career with Wrexham in 2008.
He then had spells at Doncaster and Wolves, leaving Molineux in 2013 after just four months in charge following relegation to League One.