The Japanese international, who is at the Asia Cup, has a release clause of £51 million.
Per Mundo Deportivo, Kubo is emerging as a possible attacking option for United in the summer.
The Red Devils are not planning any spending in the winter transfer window this January.
Liverpool, Juventus and Saudi side Al-Nassr are the other teams chasing after Kubo.
However, the Japanese star is happy at La Real and the club will try to convince him to stay.
