Tampa Bay Buccaneers, the NFL team run by the Glazer family who also own Manchester United, suffered a TV "blackout" on Sunday, their 17-14 victory over Randy Lerner's Cleveland Browns prohibited from being broadcast on local television because they failed to sell all the tickets. The blackout rule is aimed at ensuring stadiums are full, yet for that first game of the season, just 41,554 fans turned up, a record low in the Bucs' 12 years at the 65,900-capacity Raymond James Stadium.

That acreage of 24,000 empty seats, an unprecedented expression of supporter disillusionment, is blamed by many on the Glazers' reduction of investment in the team, who won the Super Bowl in 2003, made the play-offs in 2005 and 2007, yet have since slumped, finishing bottom of their division last season with a dismal 3-13 record (ie won three, lost 13).

The underinvestment is continuing this season, according to wage spending figures filed with the NFL players' union, which the Guardian has seen. They show that despite booming NFL TV income of $3.3bn (£2.14bn) a year, which is shared equally between all franchises, the Bucs are spending $84.4m (£54.75m) on players' salaries, the NFL's lowest, dramatically less than their competitors.

Joel Glazer, the Bucs' co-chairman and a United director, has acknowledged the spending cuts, but in an echo of the recently declared policy at Old Trafford, said this is due to a policy of signing young players, not established stars.

"We are building through the draft," Glazer said in an interview with the Tampa Bay Tribune. "That's where we are in the cycle. There were many years when we were near the top of the league in spending. Then there's the backside, where we are now; then you go back towards the top."

Yet many Bucs fans repeatedly link the underspending to the takeover of United, bought by the Glazers in 2005 with borrowings of £559m, which were then loaded on to United to repay. Since then, England's highest-earning club has poured out an eyewatering £400m in interest, bank fees and charges, yet still the total debt has mushroomed, to £716m.

Some campaigning United fans point to Tampa Bay and argue that decline, underinvestment in players and the melting away of a 100,000-strong season-ticket waiting list to leave empty seats, carries a dark warning for what is beginning to happen at Old Trafford. Andy Green, a financial analyst and United supporter, investigated the Glazers' only other significant business, the property company First Allied, finding it severely hit by the economic downturn, with 29 of its 64 US shopping centres struggling to pay their mortgages, and nine gone bust or into mortgage default.

"The Glazers do not appear to have the money from anywhere else to pay the debts at United," he says. "The NFL salary figures show the Glazers happy to invest far less in players than their competitors, and that has worrying implications for United."

The most up to date NFL salary figures collated by the players' union show teams spending on average $124m in salaries for this 2010 season. Yet despite the NFL's level financial playing field, with income from merchandising and ticket sales also shared to encourage even sporting competition, the Bucs are spending fully $40m, 32%, below that average.

This is the first year the NFL has no salary cap, due to a standoff between team owners and the union, so there is neither a maximum a team can spend on wages, nor a minimum above which teams have to spend. The figures show the Bucs spending way below their competitors in the National Football Conference South: the New Orleans Saints, the highest spenders, have a $148m salary bill; the Atlanta Falcons are paying $120m, and the Carolina Panthers' $113m is still $29m, 34%, more than the Bucs.

"The Bucs are spending less than every other team, yet all teams make the same money," explains Peter Schaffer, principal of All Pro Sports & Entertainment, which represents NFL players. "That is their choice, but US sports fans are mostly blue-collar workers, who will vote with their wallets if they feel owners aren't giving the team a chance to compete."

For some fans, the uncomfortable portents are there. For the first time since United expanded Old Trafford to its current 76,000 capacity in 2006 – done with cash built up before the Glazers took over – the club has not sold all its season tickets, falling 2,200 short of the 54,000 target. David Gill, United's chief executive, arguing that was "pretty good in the current climate", appeared to blame the recession, rather than the "green and gold" protests against the Glazer regime or last season's lack of success.

Both Gill and Sir Alex Ferguson have sought to maintain the unconvincing line that investment in the team is not affected by the £716m debts, 16.25% interest rates on the £202m "payment in kind" loans included in that, and huge annual interest paid, £42m last year, which has put United into loss every year since the Glazers took over except 2008-09 due to the exceptional £81m received for Cristiano Ronaldo.

Since United were outclassed in the 2009 Champions League final, defeated 2-0 by a Barcelona team which included five players who went on to feature in Spain's World Cup winning side this summer, Barcelona have added another Spanish World Cup winner, the striker David Villa, and Javier Mascherano from Liverpool, to their squad. Ferguson, claiming he still has the world to choose from, has since then sold Ronaldo and allowed Carlos Tevez to leave, signing Antonio Valencia, Michael Owen, Gabriel Obertan, Chris Smalling, Javier Hernández and the £7.4m Bébé, whose only previous competitive matches were in the Portuguese third division.

The explanation from Old Trafford echoes the one emanating from the Raymond James Stadium: Ferguson says he has money to spend, including the Ronaldo proceeds, but does not see sufficient "value", or Manchester United-quality players around, and wants only to invest in young talent.

Paul Stewart, a London-based Bucs fan who runs the www.bucpower.com website, still greatly credits the Glazers for the rebuilding which led to the Super Bowl, and believes this frugal rebuilding will bear fruit. "They are business people," he says.

The Glazers bought the Bucs at a shrewd time, 1995, as the NFL was beginning to boom financially on Fox Network's then record $1.6bn four-year TV deal which broke CBS's 38-year dominance. That was a similar landmark to BSkyB, another Rupert Murdoch TV company, enriching the Premier League from 1992 by outbidding ITV. Stewart points to the Bucs' valuation now, $1bn according to Forbes magazine, compared to $192m the family paid for the franchise. United, despite the debt, is also acknowledged to be worth more than the £790m they paid for the club that fateful May in 2005.

Yet even Forbes, to whose valuations the Glazers have pointed for affirmation of their business strategy, is having its doubts. Downgrading the Bucs out of the top 10 highest earning teams last season, Forbes said of the Glazers' Bucs, United and First Allied collection: "Revenue has thus far been sufficient to cover debt service obligations. But the situation is tentative."