His success has led to talk of a move, with AC Milan understood to hold a strong interest.
However, Castro has said that he is "calm" about the future of his manager.
"In recent days there has been talk that there are teams interested in our coach. Obviously nobody is bitter [about this]; you try to have a good season and he made this happen, but I said a few weeks ago I talked him about this," he told SFC Radio.
"I am calm and he is also. When we return to Seville [from the LFP World Challenge in Indonesia], he will sign his contract, as we had anticipated. I have no reason to think anything else because he is at ease in the Seville, as he has always said to me and it is normal that coaches to aspire for more.
"The contract will add at least one more season and I hope next year we equal or better this season, in the same way that we say to players who are coming here."
Milan will reportedly replace Clarence Seedorf as head coach this summer.