Baron Capital has been buying up stock and now owns 24 per cent of all the United shares that were sold by the Glazer family in 2012, though that still only represents 2.5 per cent of the club.
Analysts believe the accumulation of shares reflects the firm viewing United as a good long-term investment, rather than Baron Capital making any bid to secure a controlling interest in the club.
The Glazer family only released 10 per cent of the club on to the stock market - they still retain 90 per cent and control the vast majority of votes.
Nevertheless, Baron Capital's actions explain why the share price has been slowly rising despite the club's struggles on the pitch. This has also been fuelled by expectation of an announcement of a new Nike kit deal worth hundreds of millions of pounds.
Baron Capital's United shareholding was confirmed in a securities and exchange commission filing on the Manchester United investor relations website. It says Baron Capital now owns 9,581,636 shares, which were worth 151million US dollars (£90million) at the close of the market on Monday.
Baron Capital says on its website the company remains positive about United's future.
It said: "Shares of Manchester United dropped... due to a delay in the signing of a new global merchandise deal with Nike and the team's poor performance on the field. The Nike deal is still expected to be signed, but has been pushed out from this fiscal year. We remain positive on the company's prospects going forward."
London-based financial analyst Andy Green, who writes a blog on football finance and advises the Manchester United Supporters' Trust, said the move by Baron Capital appeared to be purely an investment strategy and was not the start of a takeover bid.
Green said: "This is absolutely not the start of any takeover. You could buy all the shares available on the stock market and you would still only have 10 per cent of the club and about 1.3 per cent of the votes.
"I am sure they are looking at the investment opportunities and rumours of the new Nike deal. They have a reputation as a long-term investment company."
The United share price closed at 15.84 US dollars on Monday - the highest level since November.