The Black Cats – sitting bottom of the Premiership after 11 games - are still £40million in debt and say they have already thrashed out a deal with their lenders to pay the money back at a lower rate.
The club's first financial report since de-listing from the Stock Exchange today revealed an £8.8million loss in the year ended July 31, due to investment in promotion.
Chief executive Peter Walker said: "We have invested the maximum possible in the squad and, despite a difficult start, it is vital that the club and its supporters continue to give the team all the support possible for the rest of the season.
"There is no reserve of money waiting to be spent in January (when the transfer window re-opens).
"Any additional spending will have to be funded through increasing income in areas such as retail, match-to-match ticket sales, part season-ticket sales, corporate sales, and so on.
"And it is up to Mick (McCarthy) whether he wants to move people out to bring people in."
Walker said this would come as no surprise to the manager.
"Mick is fully aware of what the club had available to spend," he said.
"Since promotion, all available resources have been directed to team strengthening, including additional funds made available by lengthening the repayment periods for our two bank loans.
"The board, manager, players and staff are fully committed to the objective of staying in the Premier League.
"It is important that everyone connected with the club stays positive and continues to give Mick and the players our support as they work very hard to progress."
Sunderland fans will not be pleased with today's comments with many losing hope in top-flight survival.
But the club can hardly be accused of penny-pinching after maintaining a wage bill of more than £16million last season – double that of all but four other Championship clubs.
Including wages, transfer fees and other costs, the club says it has spent more than £10million on the squad since promotion, but that has brought a return of only five points so far this season from 11 games.
Walker said: "We are all disappointed with how it has gone so far this season, but that is not a criticism of Mick.
"He is just as disappointed. We always knew it was going to be a tough year, but it is turning out to be even harder than we expected."
Sunderland's turnover in the year ended July 31 was £25.45million, down from £28.531m the previous year. This was mainly due to having les income from play-off and cup games.
Pre-tax losses were up from £1.26m to almost £8.8m, but the debt remained stable.
The Stadium of Light is now valued at £85m.
Chairman Bob Murray was not available today but will be speaking to the Echo exclusively next week.