The club posted losses before tax of £312.9 million over the last three years.
According to a letter sent to The 1958 fan group, the current level of losses is unsustainable for the club, which has led to cuts in every department.
INEOS chief Sir Jim Ratcliffe has implemented cost-cutting measures, including reducing the salaries of club ambassadors Denis Irwin, Andy Cole, and Bryan Robson, and even firing Sir Alex Ferguson from his ambassadorial job.
To avoid a PSR breach, United may need to sell an academy graduate, with Alejandro Garnacho heavily linked to Chelsea this month.
The club has also addressed fans' concerns that cutting losses could lead to increased ticket prices, admitting that was the case.
A letter sent to The 1958 and FC58 Coalition read: “As previously communicated, we are however currently making a significant loss each year – totalling over £300 million in the past three years.
"This is not sustainable and if we do not act now we are in danger of failing to comply with PSR/FFP requirements in future years and significantly impacting our ability to compete on the pitch.
“We will get back to a cash positive position as soon as possible and we will have to make some difficult choices to get there.
"That has included a significant reduction to our workforce as well as cuts across many areas of spend across our club.
“None of this has been easy, but we believe it is essential to restoring financial sustainability to the club which will underpin us as we work to get back to the top of English and European football.
“We do not expect fans to make up all the current shortfall – but we do need to look at our ticketing strategy to ensure we are charging the right amount, and offering the right discounts, across our products for our fans.”