Oblak, 26, signed a new contract at Atletico in April, but his buyout clause was increased by only €20m to a figure the United hierarchy believe would still be excellent value for a player considered one of the world's best.
De Gea, 28, was United's player of the year in four of the last five seasons, but made the latest mistake of an error-strewn campaign in Sunday's 1-1 Premier League draw against Chelsea and has one year left on his contract.
Oblak has had the opportunity to join Liverpool in the past, while Chelsea have also shown an interest. The Slovenia international had previously always preferred to stay at Atletico.
Despite extending his deal until 2023, with a pay rise included, sources told ESPN FC Oblak is in a similar situation to teammate Antoine Griezmann in that he would like to see more top players arrive in order to help them compete with Barcelona, with Real Madrid also likely to strengthen.
United are considering whether to pay Saul's €150m buyout clause, while Manchester City and Barcelona are interested in Rodrigo, who has been likened to Sergio Busquets.
There is an understanding that Oblak would only leave Atletico for a team playing in the Champions League -- something which is far from certain at United, who are sixth with two games remaining after taking just one point from the last nine.