The defensive midfielder is contracted to LDU Quito and was capped by Ecuador at U17 level when just 15.
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Castillo, now 16, has been tracked by United scouts and has tasted senior football with LDU Quito.
United are now firming up their interest, reports Bolavip, though they're not the only European giant watching the midfielder.
Bayern Munich and Atletico Madrid are also keen on Castillo, who cannot leave for Europe until he turns 18.
Interestingly, it's been suggested scouts from United are now attending regularly LDU training sessions to keep tabs on Castillo.
In United's favour is the connections LDU have with former club captain, Antonio Valencia, who could have an influence on Castillo's final decision.