Argentina defender Rojo is currently the subject of disciplinary action by the Portuguese club after apparently refusing to train amid reports of a £16 million bid from United.
Management company Doyen Sports, which owns 75 per cent of the player, has now hit back over Sporting's stance and threatened legal action if fees due to it from any rejected bid are not paid.
It is reported that Doyen is entitled to a payment from Sporting worth 75 per cent of any declined offer for Rojo. In this instance that would equate to £12million.
In a lengthy statement, Doyen Sports has stressed it cannot interfere in transfer business and that Sporting have a right to retain the player.
But it has pointed out that in such a scenario the club still have obligations to the company.
The statement read: "Sporting is entirely within its rights not to transfer the player Marcos Rojo, knowing that it only has to make up for the fund under the terms and deadlines as contractually established since the beginning."
It added the company "will not hesitate to use all legal resources at our disposal to defend fully all our interests and rights".
Doyen holds strong rights over the player after paying 75 per cent of his transfer fee from Spartak Moscow in 2012.
The statement added: "Without the intervention of Doyen, through financing, Marcos Rojo would not be a Sporting player."
Sporting have responded with their own detailed statement which alleges interference from Doyen.
Marcos Rojo has everything it takes to be a Man Utd hero
They insist they have had "just cause" to terminate their agreement with the company, therefore suggesting they do not intend to pay any fees.
Sporting, who are reported to be holding out for a figure closer to the player's £24m release clause, have also confirmed that Spartak are owed 20 per cent of any fee received for Rojo above £4m.
- Sports & Recreation