The Red Devils have had a poor Premier League campaign and part of the blame should go to the defensive set-up. Onana has been guilty of making multiple errors.
It is now reported that United are planning to pursue a new shot-stopper. Onana could make way for Santos, who has made immense progress with Frankfurt of late.
The 22-year-old has stepped up in the injury absence of Kevin Trapp lately and he could become the permanent number one due to his fine displays.
The same publication claim that Frankfurt have no plans to part ways with their prized asset unless they receive around €60 million on the table.
United are unlikely to afford such a fee but the scenario could change if a Saudi Pro League outfit were to pay big money to purchase Onana this summer.