The manager?s sibling was in the crowd for Rayo?s 3-1 win over Getafe on the weekend and, although the Brazilian failed to get on the scoresheet, he did provide the assist for his side's third and final goal.
United could firm up their interest safe in the knowledge that the 20-year-old would be available for as little as ?1.6m.
With Reading keen to take Danny Welbeck on loan and United already lending out teenage striker Angelo Henriquez to Wigan there could be room for Baptistao on the United bench.
The exciting striker has been playing as a lone striker for his La Liga side and has hit six league goals in 17 appearances.
The forward played in the same youth team as Brazil sensation Neymar and was tipped for great things but illness and injury have hindered his progress up until now.