One close watcher of United's finances, who wished to remain anonymous, said sales were thought to be "5,000-10,000 down on last year".
He said this was down to discontent with the Glazer regime and the wider economic situation but it is difficult to say if there really is a link between the fan discontent and the issue of ticket sales.
It is also unclear how many of the club's 8,500 premium seats - 7,000 executive seats and 1,500 hospitality packages - have been sold.
Over 40% of Manchester United's match-day revenue comes from these premium seats, despite them making up only 11% of Old Trafford's 76,000 capacity.
The need to place season tickets on general sale - seven weeks after the deadline for renewals had passed - also raises questions about the club's much-trumpeted waiting list, an untapped reservoir of support supposed to number 60,000 fans.
This is an issue that is likely to come up with investors in August when the next set of quarterly accounts are due.
The three-monthly reporting of the club's financial results was a condition of this year's controversial £504m bond issue and chief executive David Gill could experience an uncomfortable bondholders' conference call.
But nobody can accuse the Premier League giants of not trying hard enough to shift their tickets.
We are very happy with the way sales have gone and have already sold more season tickets than the capacity of 17 Premier League grounds
Manchester United spokesman
Anecdotal evidence suggests there has been an unprecedented sales drive this summer, with particularly attractive deals offered to wavering executive-seat customers.
A spokesman for the club told BBC Sport that there was nothing unusual about the decision to place season tickets on general sale and claimed sales were in line with expectations.
"There is always 'churn' of season-ticket holders who don't renew for various reasons. The churn was within the normal tolerances when compared to recent seasons," he said.
"Because of the shortness of time between now and the start of the season, we have to go on general sale at the same time as the waiting list is considering its response or we would lose time. This is in line with previous year's.
"Executive sales are exactly in line with last year, which means they are slightly ahead of 2008 at the depth of the recession.
"As a club, we are very happy with the way sales have gone and have already sold more season tickets than the capacity of 17 Premier League grounds."
The backdrop to this story is one of booming ticket sales at Manchester City - albeit from a much lower base - and solid sales performances from rivals such as Arsenal.
In June the Red Knights, a group of wealthy supporters mulling a takeover of United backed by the "Green and Gold" movement, put plans to make a bid for the club on hold.
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