Mr Bellew was the second in command at Malaysia under Mr Müller, who had departed as boss of Aer Lingus in 2014 to help turn around the troubled Asian carrier.
Mr Müller’s unexpected resignation comes as the carrier is beginning to turn around after two plane crashes in 2014 dented its sales and reputation, prompting the government to buy out all shareholders and take the company private.
The Irish Examiner profiled Mr Bellew as he prepared to take up the job as chief operations officer at Malaysia last September.
A native of Bettystown, Co Meath, he lived outside Killarney.
His current job at Malaysia made him responsible for all operations at the airline, including engineering, flight operations, and ground operations, as well as having responsibility for the pilots and in-flight crew across a fleet of 140 planes.
He had been a senior director helping to manage Kerry Airport in the 1990s.
Malaysia had poached Mr Bellew from Ryanair, where he had responsibility for pilots at the airline, and the punctuality of the airline’s services.
Friends at the time said that though he was leaving to join a much larger airline, he was looking forward to the challenges of helping to turn around deep problems at Malaysia.
He started his career as a travel and air travel specialist — dealing with Ryanair—about 30 years ago.
He later joined Kerry Airport where the passenger numbers increased from 6,000 to 400,000.
He had worked for Ryanair for nine years and managed legal disputes with pilots. Previously, he was responsible for sales and marketing.
“The show must go on,” Mr Müller wrote to employees Tuesday, informing them he was leaving the carrier in September.
“We have delivered our financial budget for the first three months of 2016, our on-time performance is increasing and very stable and our customer service index is recovering with more significant improvements in the pipeline,” he said.