The Daily Mail says United are scouting Blopa, 18, this season.
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The teen can play across the defensive line and is known to United's former Sporting coach Ruben Amorim.
Real Madrid are also watching Blopa's progress, who is now a first teamer under coach Rui Borges.
Blopa scored twice on his debut in the Cup win against Alverca, becoming the first player to do so at Sporting since 1938.