The 32-year-old has made just one league appearance for the club this season after picking up a hamstring strain, which was operated on last Thursday.
"I feel good. The surgeon who operated on me said it all went well. It is not bothering me at all so we'll wait and see how it goes this week and maybe I can start next Monday," he told reporters at a press conference.
"I need to wait until 10 days after the operation and if all is well I think I'll do some swimming before joining up with the rest of the squad."
Castro joined Betis from Deportivo La Coruna in 2010.