Cayetano is currently in southern California waiting for a matching liver donation after tests showed his body was bound to reject any liver donation from his family.
De Castro said Cayetano sounded upbeat on returning immediately "although his voice has already changed."
"I told him that all of our colleagues in the Senate are wishing him speedy recovery and are equally eager to see him back," De Castro said.
He said he would definitely vote for Cayetano as Senate president the moment he fully recovers from his present ailment and returns to the Senate.
Cayetano’s ailment has prevented the implementation of his term-sharing agreement with Senate President Franklin Drilon. He was supposed to take over from Drilon at the resumption of the session last Monday.