Lee Harrison is set to leave Wycombe Wanderers and join a Premier League club as their technical head of goalkeeping.
The 42-year-old, who will depart after Saturday's game against AFC Wimbledon, arrived at Adams Park in June 2012 and has since played a pivotal role in coaching both the goalkeepers and outfield players at the club.
said: "Although he will be sorely missed by everyone, I'm thrilled for him. He's a top guy who deserves this opportunity.
"I know both Matt (Ingram) and Charlie (Horlock) have come on in leaps and bounds under his watchful eye, but his influence wasn't just limited to the goalkeepers.
"All the outfield players, and both myself and Dobbo, have all had the pleasure to work with such an enthusiastic, hardworking and honest guy."
Ainsworth added: "To make his contribution even more amazing, Lee did all this while his youngest son, Luke, had to have a kidney removed due to cancer. I cannot begin to comprehend what he went through but his, and Luke's, strength is something we all came to admire.
"Myself and Dobbo have had some fantastic moments with Harry and rather than losing a colleague, we've gained a true friend."
said: "I've had a fantastic 18 months during a tough time for me and my family which says a lot.
"The support of Gaz, Dobbo, Gary Waddock, the staff and the fans has made that time so enjoyable and I won't ever forget it.
"I'm sad to be leaving Gaz and Dobbo, my two best friends in football, but it's an opportunity that I couldn't turn down."
He added: "I'm delighted that I've been able to help develop Matt and Charlie, who are both superb young goalkeepers. Matt is now a first-team regular and Charlie has come on massively.
"I wish them, the players, staff and supporters all the best."
Wycombe Wanderers would like to thank Lee for his efforts during his time at the club and wish him and his family all the best for the future.
The club are seeking a replacement for Lee and an announcement will be made in due course.