Shares in Manchester United fell in New York on Wednesday after the resignation of Sir Alex Ferguson's retirement.
The news was announced while US markets were closed. When trading began the club's shares fell as much as 4.5%.
Before its stock market flotation last year, the club said in its prospectus that its business was dependent on its ability to attract and retain players.
"Any successor to our current manager may not be as successful as our current manager," it warned.
Shares eventually closed down 1.8%.
H20 Markets chief equity strategist Michael Jarman, a former professional footballer, said Sir Alex has been an integral part of the club's commercial success.
Speculation is already mounting as to who will succeed Sir Alex, who won 38 trophies during his 26 years in charge at Old Trafford.
The BBC reports that whoever gets the job will join a club laden with almost £370 million in debt and tightly controlled by the Glazer family which bought the club for £790 million in 2005.
Since its flotation in August 2012, Manchester United shares have risen 34%.
The club's value is more than $US3 billion, making it the second wealthiest football club in the world.
The BBC reports Manchester United gets about a third of its revenues from match ticket sales, a third from broadcasting deals, and a third from commercial business.