Marco Silva resigned as coach of Greek champions Olympiakos Piraeus on Thursday with the club immediately announcing Spaniard Victor Sanchez as his successor.
The 38-year-old Portuguese Silva, who led Olympiakos to a record 43rd league title last season, cited personal reasons for his sudden and unexpected departure.
"Olympiakos announces to its fans that the coach, Marco Silva, has tendered his resignation due to personal reasons. His resignation was accepted," the club said in a brief statement.
Olympiakos then published a second statement, adding: "We further announce that we have reached an agreement with Victor Sanchez to be the club's new coach."
The 40-year-old Sanchez, who worked at Olympiakos as assistant to former coach Michel in the 2013/14 season, managed Deportivo La Coruna last season, but was sacked after the club finished 15th in La Liga.
Silva had become a hugely popular figure with the Olympiakos fans after enjoying a record-breaking first season in charge in 2015/16, wrapping up the title with six matches to spare and chalking up a club-record 17 league wins in a row in the process.
According to local media reports, Silva informed Olympiakos's shocked players of his decision on the first day of their pre-season preparation camp in Austria.
Silva had been linked with FC Porto before they appointed Nuno Espirito Santo.