"Time will tell if it's possible or not," said Castro when asked about signing Rakitic.
He also discussed strengthening the team with some players if there was a need.
"[Signings] is for the winter market and we have to be focused on winning matches," he said. "We have already made a very big effort this summer.
"Signings are Monchi's thing. It's not about who wants to strengthen the team. We have a great team and we have to think about our options.
"When the window arrives, if there is any option that Monchi likes, it is a matter between him and the manager."