Manchester United are seriously considering triggering Atletico Madrid goalkeeper Jan Oblak's €120 million buy-out clause, sources have confirmed to ESPN FC -- throwing David De Gea's future into doubt.

Oblak, 26, signed a new contract at Atletico in April, but his buy-out clause was only increased by only €20m to a figure which the United hierarchy believe would still be excellent value for a player considered one of the world's best in his position.

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 2018-19 campaign in Sunday's 1-1 Premier League draw against Chelsea, and has just one year left on his contract.

Oblak had the opportunity to join Liverpool in the past, and Chelsea have also shown an interest. The Slovenia international had previously always preferred to stay at Atletico.

Despite extending his deal with Atletico up until 2023 with a pay rise included, sources have 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 -- while Real Madrid are also likely to strengthen.

Atletico have already lost 2018 World Cup winner Lucas Hernandez to Bayern Munich, while defenders Diego Godin, Juanfran and Filipe Luis are all out of contract in the summer. Meanwhile, several top clubs are interested in other key players including midfielders Saul Niguez and Rodrigo.

United are considering whether to pay Saul's €150m buy-out 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, with Ole Gunnar Solskjaer's team sitting sixth with two games remaining after taking just one point from the last nine available.