Millwall assistant manager Marc Bircham has left the club for a post with Major League Soccer side Chicago Fire.
The 36-year-old, who began his playing career with the Lions, rejoined the Championship club in January last year.
"This was a very hard decision to make because I really feel there are good times ahead for Millwall," he said.
"But I had almost gone to the MLS earlier in my career and this time it just seemed like such a great opportunity for myself and my family."
First-team coach Nigel Gibbs is set to become Ian Holloway's assistant following Bircham's departure from The Den.