Manchester United shares fell in New York amid concerns over the impact Sir Alex Ferguson's retirement will have.

The news was announced while US markets were closed. When trading began the club's shares fell as much as 4.5%.

The club said in its prospectus ahead of its stock market flotation last year 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.

The shares eventually closed down 1.8%.

Speculation is already mounting as to who will succeed Sir Alex, who won 38 trophies during his 26-year reign at Old Trafford, with Everton's David Moyes and Real Madrid's Jose Mourinho both being tipped.

Whoever gets the job will join a club laden with almost £370m of debt, and tightly controlled by the Glazer family.

The Glazers bought the club for £790m in 2005 in a controversial deal that loaded the club with debt.

The flotation in New York saw the Glazers sell 16.7 million shares, equal to a 10% stake in the club.

Since the flotation in August 2012, Manchester United shares have risen 34%.

'Risky' stock

Image caption Sir Alex leaves Old Trafford as the most successful manager in English club football history In-depth: Business of Sport

Ken Perkins, an analyst at Morningstar who covered the club's flotation, told the BBC: "When we initially did the analysis of the IPO (initial public offering) one of the concerns about the outlook was Sir Alex leaving."

He said that even if the club was to install a new manager relatively quickly, it would merely instil some confidence in the shares in the short term, whereas many investors are more interested in a company's ability to generate revenues over the long term.

Manchester United gets about a third of its revenues from match day ticket sales, a third from broadcasting deals, and a third from commercial business.

"This stock probably has a lot of speculation built into it," said Mr Perkins.

"It may not trade a lot on fundamentals, but may trade on things like who's the new manager, how the team performs.

"It's hard to say how other investors will feel. That's why in our view stock like Manchester United is risky."