Eric Ramsay has left his role as an assistant coach with Wales due to personal reasons.
"This has been an extremely difficult decision to make," said Ramsay.
"But it is the right one for myself and my family after the recent birth of our second child and taking into consideration the intensity of my role at Manchester United.
"Having discussed everything with my family, Rob [Page, Wales manager] and the Association [Football Association of Wales], we came to the conclusion that it was best to depart my role before the upcoming internationals so that the group can fully concentrate on the fixtures.
"I'm extremely thankful to everyone at FA Wales; the experiences throughout the last six months have been amongst the proudest moments of my career to date.
"It was a real pleasure to work with this group of players and I wish everyone nothing but success going forward."
A statement from the Football Association of Wales (FAW) read: "Eric Ramsay has stepped down as assistant coach due to personal reasons.
"Rob Page and the FAW would like to thank Eric for his work with the team and wish him well for the future."
Lester joined Page and his fellow coaches on Monday for a training session at Wales' training base in the Vale of Glamorgan.
A former team-mate of Page at Sheffield United and Chesterfield, Lester has held coaching positions with Nottingham Forest and Chesterfield.