The Red Devils have struggled in David Moyes' first season in charge, occupying seventh place in the Premier League and languishing 11 points behind Liverpool, who occupy the final qualifying place behind Chelsea, Arsenal and Manchester City.
With the prospect of even qualifying for the Europa League hanging in the balance for last season's Premier League champions, Purslow says the Old Trafford club face a crucial fight for the long-term financial future of the club.
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"Next year, when that BT Sport deal kicks in, that figure will probably double. In other words the difference between fourth and fifth place in the Premier League may be between £50-60m a year.
"That affects Manchester United the most. They don't look like they're going to be there this year. It gives Moyes and [chief executive] Edward Woodward a year to get their act together.
"They can live with a £30m disadvantage for one year but, in another year's time, if they miss out [again], they may never get back [in the Champions League]."
If United fail to secure a top-four spot this season, it will be the first time the club have missed out on Europe's premier club competition since 1995.