Dramatically knocked out of the League Cup on penalties by Premier League strugglers Sunderland in front of a stunned Old Trafford crowd on Wednesday and 14 points adrift of league leaders Arsenal, United fell to fourth in the latest Deloitte Football Money League behind leaders Real Madrid, Barcelona and Bayern Munich based on income during the 2012/13 season.
International financial services firm Deloitte have been compiling the table since 1997 and for the first eight years United were the biggest-earning club in the world.
However, they've since been overtaken by Spanish giants Real and Barcelona and now Bayern following the German club's impressive treble-winning season.
But Deloitte forecast United, owned by the US-based Glazer family, would reclaim third place from Bayern next year, although a failure to qualify for the lucrative European Champions League could have a damaging effect on the club's long-term finances.
"It is the first time Manchester United have dropped out of the top three but Bayern had an exceptional year," said Dan Jones of Deloitte's sports business group.
"Next year United will have the Chevrolet deal plus other new commercial deals in their figures, and the new Premier League TV deal so we are confident they will be back in the top three.
"The longer term depends in part what happens on the pitch and if they do not qualify for next season's Champions League that is probably worth 50 million euros (2.24 billion baht) directly in terms of money from TV and attendances at Old Trafford."
This year's table also records the financial rise of Qatari-owned Paris St Germain, who have seen their earnings increase by a remarkable 81 percent, the French club rising to fifth in the standings ahead of English giants Manchester City, Chelsea and Arsenal.
With Spanish clubs being able to negotiate their own individual, as opposed to collective, television deals, Real Madrid top the Money League for the ninth consecutive year, with total revenue of 518.9 million euros, followed by Barcelona with 482.6 million euro.
Bayern are third with earnings of 431.2 million euros, followed by Manchester United with 423.8 milion euros and PSG, with 398.8 million euros.
Manchester City are sixth with 316.2 million euros, then Chelsea 303.4 million euros and Arsenal 284.3 million euros.
Italian sides Juventus (272.4 million euros) and AC Milan (263.5 million euros) complete a top 10 that no longer includes England's Liverpool, who've slipped to 12th place with revenue of 240.6 million euros.