The Ecuador International has shot Brighton since his arrival from Villarreal in 2022.
Estupinan now has less than two years to walk his current deal and Talksport says that United has now opened conversations with Brighton about his signing.
AC Milan, with both cases, are also giants who need a new left -wing addition.
At United there remains fitness problems about Luke Shaw, while Milan Theo Hernandez has just sold Al-Hilal.