Goal understands that the Reds made a breakthrough in negotiations earlier in August as they chase a £16 million deal for the 22-year-old.
However, while Castro confirmed that negotiations with the Merseysiders have taken place and that the Spain international will leave if his valuation is met, he insists that no agreement is imminent.
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"If Liverpool's offer is suitable he will leave. If not, he'll stay and nothing will happen, absolutely nothing. We'll be at ease."
Castro also stated that no offers had been received for striker Carlos Bacca, amid reports of interest from Russian Premier League side Zenit.
He added: "I'm not aware of anything with Bacca. I don't listen to the rumours and they're not true, at least not that the club is aware."
In terms of incomings, Unai Emery's men are still searching for a long-term replacement for midfielder Ivan Rakitic, who joined Barcelona earlier in the summer. Valencia's Ever Banega has been touted as a possible Sevilla target but Castro is willing to be patient in his pursuit.
"The coach knows what's necessary. I'm sure someone will join before the end of the market. No rush," Castro continued. "There will be a creative player coming in midfield. Banega is an option but so are many others."