The 27-year-old "suffered a torn meniscus in his knee in training, requires surgery and will miss the rest of the season", the club announced via Twitter on Wednesday. He underwent a successful operation on the injury on Thursday afternoon.
Castro has been one of the Werkself's most reliable performers this season, scoring twice and providing seven assists in his 20 appearances, 19 of which have been starts.
Following Matchday 29's 4-0 win over Hannover 96, Leverkusen's seventh successive league win, Roger Schmidt's side are now third in the table, occupying the final automatic UEFA Champions League qualifying place. They are next in action in the Rhineland derby away at 1. FC Köln on 25 April.