There's been an air of inevitably over the 18 year-old's club future, with the board at Universidad Catolica resigning themselves to losing their young star for over a year now.
After this summer's U20 World Cup, where Castillo was watched by United scouts playing alongside Angelo Henriquez in Chile's attack, club directors are now banking on an auction to breakout in the remaining weeks of the transfer window.
Jaime Estévez, Universidad's chairman, conceded only last week: "Nicolas Castillo wants to go to Europe, he has repeatedly expressed this. We have asked him to stay this season.
"But if you see your future elsewhere, your mind is going to drift and we have to understand this.
"If he is to leave, so be it."
A powerful, athletic striker, blessed with a never-say-die attitude, Castillo has drawn comparisons in the Chilean media to PSG firebrand Zlatan Ibrahimovic. Indeed, after first breaking into the Universidad first team as a 17 year-old, Castillo was soon banished back to the reserves over "attitude problems".
And while such arrogance has dissipated, he is still prone to losing his rag on the field and has endured problems with referees in the past season.
Universidad Catolica's highly regarded coach, the Argentine Martin Lasarte, feels Castillo still needs to smooth some of the rough edges of his game and would benefit from another 12 months in the local league.
"Today, departing South American footballers are getting younger, without meeting some milestones in their development.
"Nico has played in the junior World Cup and I think he could use some more seasons in the local environment.
"But those making the decisions today are the player himself, the club and the people who represent him."
But Lasarte does believe Castillo has everything in his locker to be a success in Europe.
"He has qualities, has an eye for goal, is brave, decisive.
"His future is very promising."
Former UC and Argentina star Alberto Federico Acosta agrees with Lasarte's assessment.
"Mario Lepe (former UC youth coach) told me they had a player who would be a star and in a couple of years was going to break into the First Division. Over the years I knew he was referring to Nicolas Castillo. Then I had the chance to see him and there's no doubt that he is a tremendous striker with a future. I think that UC has to make the most of him, but I do not think he has much time left in Chile."
Local media sources say Castillo's form in Turkey led to United offering £5 million for the striker immediately at the end of Chile's campaign. But the offer fell well short of Universidad Catolica's valuation and with Inter Milan also in contact, Estevez and his board are confident of getting their price.
Universidad sports chief Jose Maria Buljubasich reflected the board position last week, saying: "Nico has the expectation of being sold and we understand his desire. He has normal ambitions. The club has shown (in other cases) that it is open to possibilities of selling, but we do think that players have a market value, there must be agreement between the parties and should be a formal offer."