A source with knowledge of the situation told ESPN that NYCFC has not received any official offer from River as of yet. However, sources add that the Argentine club is speaking with Castellanos' agent to put together an offer for the 2021 MLS Golden Boot winner.
Argentina outlet Olé reported on Saturday that the search for Castellanos is directly related to the sale of Julian Alvarez to Manchester City, who is expected to be transferred for a reported fee of $27.5 million.
Sources have told ESPN that NYCFC values Castellanos for between $15m to $20m. Brazilian side and Copa Libertadores champions Palmeiras made a previous offer of about $12.5m, which the MLS team rejected.
Earlier this month, Castellanos told Argentina's TyC Sports that there had been contact between him and River.
"River contacted me and my agent. They asked me if I wanted to come. If [River manager Marcelo] Gallardo calls you, he's one of the best coaches in the world, you can't say no. But it's not up to me, you have to evaluate everything. It's great that someone from Gallardo's coaching staff calls you and is interested. We're monitoring, the door is not closed at all," Castellanos said.
"I spoke to a Gallardo's assistant. We gave him a contact from my representative so that they could talk. They asked me how my situation was at the club and if I was interested. Obviously, I said yes. You can't say no to River Plate, yet we're taking it as it is. A lot of things need to be seen, let's see what happens."
"It's beautiful that people talk so much about me in such important clubs in South America. They also talked about some clubs in Europe. I have to keep a cool head. I have a five-year contract with the club, I want to grow as an athlete. A boy always dreams to play in Europe, but I don't rule out River and Gallardo. Palmeiras is also the last champion of the Copa Libertadores. It doesn't depend much on me, but on the clubs."