A host of clubs had been keeping close tabs on the 25-year-old Croatia international, who has had a brilliant campaign with Sevilla this term.
"It is an important negotiation. We've already had two meetings and I believe we've come to a friendly agreement," Castro told Estadio Deportivo.
"Both Rakitic and Sevilla have made a big effort to reach this agreement.
"I don't think there will be any problems in signing the contract in the next few days, one or two details are missing, but the agreement will undoubtedly reach a good conclusion.
"The agreement is clear, we have reached an understanding. There are details to refine but they will not stop Ivan from signing a contract for the next few seasons."
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