The 22-year-old who has attract many suitors around European clubs and Red Devil have shown strong interest in the Spain international who has a £134m release clause written into his contract.
According to Don Balon Manchester United coach Jose Mourinho is willing to use Herrera and Mata in a swap deal in order to land Niguez.
Atleti boss Diego Simeone is not so keen on the transfer, however, with the same report suggesting that the Argentina coach is standing firm and will not sell at any cost.
Saul, who only agreed a new contract at the Wanda Metropolitano in the summer, has featured in all 12 of Atletico games in all competition so far this term.
