United are in the market for a new keeper amid doubts over the future of senior pair Andre Onana and Altay Bayindir.
Vanja Milinkovic-Savic (Torino), Mike Maignan (Milan) and Senne Lammens (Royal Antwerp) have all been linked with United in recent weeks.
Now BILD reports United is showing interest in Santos.
United director Christopher Vivell has already been in contact with Eintracht Frankfurt regarding the goalkeeper. Tottenham have also been tracking Santos and technical director Johan Lange is another who has contacted Frankfurt.
However, Frankfurt sporting director Markus Krösche has insisted that Santos is not for sale.
"We will definitely not sell Kauã this summer," he stated earlier this week.
Santos, 22, has a deal with Frankfurt to 2030.