In typical winter transfer window fashion, another transfer is holding up proceedings and could effectively end any chance of the deal going through this month.
And according to the Daily Star, the club have refused to cut the deal short.
With the race for the Scudetto hotter than ever and a Champions League round of 16 tie against Tottenham still ahead, the Bianconeri don't want to reduce their squad.
It was reported on Tuesday that Chelsea were prepared to offer £50m for Sandro, who is said to be keen on a move to England.
United are also keen on signing the Brazilian. Daley Blind, Luke Shaw and Ashley Young have all failed to establish themselves at left-back and Red Devils boss Jose Mourinho is looking for a permanent option.