Community leaders, unions and churches are backing a call for the club to increase the pay of workers employed by contractors, such as waitresses and cleaners.
The club said it pays permanent employees the Voluntary Living Wage, which is higher than the statutory national figure.
Manchester Citizens will hand in a letter on Thursday to the club, urging it to take a "community-first business approach" to ensure the cost of living is met for low-paid workers.
The group, part of the Citizens UK charity, said if new signing Alexis Sanchez is paid £400,000 a week, as some have reported, it will take him just 82 minutes in a game to earn the annual salary of a low-paid stadium worker.
Sanchez's pay, and millions reportedly given to his agent, reveals a "grotesque tale of two halves", with some staff on £7 an hour, said the group.
Group chairman the Reverend Ian Rutherford, City Centre Minister at Methodist Central Hall, Manchester, said: "As the winter transfer window closes many workers at Old Trafford will be choosing between putting the heating on or a hot meal.
"We've heard many stories about the real cost of life on low pay for workers at Old Trafford, many of whom are employed directly and struggling to live with dignity.
"We are calling on Manchester United, which already makes such a positive contribution to the UK economy, to show leadership by recognising the asks of the local community and acknowledging that fair pay can go a long way to improve the lives of their employees at the other end of the pay scale."
A Manchester United club spokesman said: "Manchester United pays its staff competitive salaries for the jobs they undertake.
"We have many variations of contracts in place due to the size of the club, although all permanent employees, whether engaged on a full or part-time hours basis, are paid the Voluntary Living Wage, in line with the Premier League agreement.
"Staff welfare is very important to the club and we consistently score in the top quartile of employers measured in independent surveys on staff satisfaction."
Neil Jameson, executive director of Citizens UK said: "Chelsea, Everton and West Ham are setting the standard on low pay by accrediting with the Living Wage Foundation and paying a higher, voluntary rate. We would urge all fans and their communities to get behind our campaign for all Premier League Clubs to pay a real living wage."
:: The voluntary wage is £8.75 an hour outside London and £10.20 in the capital, compared with the compulsory National Living Wage of £7.50 for over 25-year-olds.
Before the Emirates FA Cup last week, Manchester United players stood for a minute's applause for Christian football legend Jimmy Armfield, who recently died at the age of 82.
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