Goalkeeping coach Paul Gerrard has left Latics to link up with former manager Paul Dickov at Championship club Doncaster Rovers.
Gerrard (pictured), who played as a professional for Latics between 1991/96, returned as goalkeeping coach three years ago.
He also had a spell as joint caretaker manager with Tony Philliskirk following Dickov's departure in January.
Manager Lee Johnson says 40-year-old Gerrard leaves with his good wishes, though he admitted his departure is a setback.
He explained: "It is the first disappointment since I have been here.
"I am disappointed to lose Paul as we got on well and I enjoyed working with him.
"There are no hard feelings and I wish Paul all the best at Doncaster which is a great club."
Johnson added it is important to go into the season with a settled backroom team.
"I am shrewd and have made provisions to bring somebody in as good as Paul, if not better," he said.