I think La Liga and Serie A clubs should think twice about signing Onana, as he no longer possesses the same abilities as he did at Inter. His quality has clearly declined, so rather than waiting for clubs from other leagues, it's better to accept offers from Turkey.
What if we should think deeper? How many players were able to succeed at Manchester united lately? Why is it that some of them transform into their prime when they left Manchester united to another club? What if the problem is within the Manchester United system and not from the player's side? The Onana I know is a good goalie that I wish my club signed him that year but Manchester united were the lucky team to got him and all of a sudden, he started behaving abnormal making silly mistakes and became part of daily funny memes.
Yes, it's true that players who leave Manchester United tend to have career upturns. We've seen this with players like Rashford and Antony. However, when we look at Onana, he was a goalkeeper who made a lot of individual mistakes. He caused Manchester United a lot of problems last year. Therefore, perhaps a loan move to another team this year would be a good way to revitalize him. If a deal is reached for him, he'll join a good team in Turkey.