Sir Alex is widely touted as one of the best football managers of all time, with 38 trophies and 13 league titles in his 27 years with the club.
From a business perspective, the question is where to now, for the financial viability of the club, and company?
Manchester United was listed on the New York Stock Exchange in August last year, in a bid to help rid of the debt creeping up on its books.
The company picked the NYSE over other exchanges because of the various start up incentives offered in the US, which would keep IPO, or sharemarket listing costs down, in a bid to grow investment in the sector.
Outside of the Glazer Family stake, the biggest shareholders of the club are Baron Capital Management, Soros Fund Management and Blackrock .
Manchester United's latest accounts show that it posted a record $5.5million in the three months to March, up on the $1.5million profit on the same time last year.
Total revenue for the quarter came in at $145million.
New sponsorship deals helped it to increase commercial revenue by 32 per cent in the period, while broadcasting revenue surged 28 per cent.
Match day revenue, which includes ticket sales rose 28 per cent.
Those numbers helped shares in the company grow by 31 per cent since the beginning of the year, giving it a market capitalisation of around $3billion.
But what will investors think now, that one of its biggest risks to profit, is gone.
In its IPO prospectus, lodged with the United States Securities and Exchange Commission, the company stated under the "Risk Factors" category that, "Any successor to our current manager may not be as successful as our current manager. A downturn in the performance of our first team could adversely affect our ability to attract and retain coaches and players."
How Sir Alex's retirement may impact investor sentiment is yet to be clearly seen, while others have no doubt already factored it in.
What is obvious, is that his successor will not only need to appease the club's fans, but shareholders as well.
Manchester United shares closed the session -1.7% at US$18.44.
