Besides Atletico Madrid would also not like to lose one of their most valuable players. Oblak is still a great goalkeeper. I don't see any reason for Oblak to choose to join Manchester United now either. Atletico Madrid are in much better condition now.
This will remain a rumor in my opinion. Oblak is doing very fine at Atletico Madrid and he is a keeper that I’m sure Atletico Madrid will not want to sell now for anything. Unless if and only if, Oblak has interest in playing for Manchester United and he has been given a good offer. Come to think of it again, why would Manchester United go for a keeper of that age and add to Onana that is already with them?
I feel it would have been better for them to get a younger goalkeeper than they can groom for many years to come in the club, just like the case of De Gea who have left them now recently. Signing Oblak will cost them a lot seeing that he already signed a new contract with his current club which shows how loyal he is and this will pose a lot of competitive disadvantage to Onana who is already their number one goalkeeper. I just have a strong feeling that this is just a rumor and such will not happen.