If Hull win they will stay up - and send two of the three North East clubs down, costing them £30million apiece in lost revenue.
Paragraph E20 of Premier League rules states: "In every league match each participating club should field a full strength team."
No club has ever been cited for breaching the rule. But officials from Newcastle, Boro and Sunderland have discussed taking action if United send out a shadow side in the final game of the season.
A source close to the talks told The Sun: "United have every right to think of themselves but that shouldn't be to the detriment of others.
"If they field a reserve team at Hull who then win and stay up, then the consequences for two of the three North East clubs will be dire.
"No one is looking for a favour, just a level playing field. And if the rules are not adhered to there are means of recourse."
A Premier League spokesman said: "We are aware of this situation and we will be monitoring all matches on the final day of the season."