The Red Devils signed Onana from Inter Milan in the summer of 2023, and the Cameroonian has made 55 appearances for the 20-time English champions in all competitions.
Onana has found it difficult to show his best form for Man United, though, conceding 89 times and keeping only 14 clean sheets, and another tough season for the 28-year-old would leave the English outfit with a big decision to make when it comes to their goalkeeping position.
Altay Bayindir also arrived last summer, but the Turkey international is viewed by the club as a number two, meaning that an exit for Onana would almost certainly see the Red Devils enter the market for a replacement.
According to Fichajes, Man United are 'very interested' in Simon, with Erik ten Hag's side huge admirers of Spain's Euro 2024-winning goalkeeper.
Man United 'keen' on move for Spain goalkeeper Simon
The report claims that the Red Devils are prepared to pay 'a considerable sum' to bring in Simon, who has been in the Athletic first team since 2018, making 195 appearances for the club in all competitions.
The goalkeeper is also first choice for Spain, earning 46 caps, including six appearances at Euro 2024, but he missed his country's two September internationals due to a wrist problem.
Simon, who is valued at £25m by transfermarkt, has a contract with the Lions until June 2029, but the 27-year-old could potentially earn himself a big-money transfer next year.
Will Man United replace Onana with Simon?
It would be a big surprise if Onana lost his spot as number one under Ten Hag, but if there is a change of manager at Old Trafford, then the stopper's future would be called into question.
Onana made some big mistakes during his first season at Man United, including in the Champions League, and he has also been criticised for his displays in the early stages of this term.
The Red Devils need the best in class if they are to return to the top of English and European football, and Onana's suitability for the club continues to be called into question.
Much will depend on how the season progresses, as the Cameroonian certainly has the capabilities of enjoying an excellent campaign, but a poor season would lead to questions being asked.
Simon is undoubtedly currently one of the leading stoppers in world football, and it would not be a surprise to see him leave Athletic for one of the world's biggest clubs next year.