The four-time chief minister arrived at the hospital for a routine check-up on Sunday. However, the doctors advised that he be kept under medical observation for respiratory problems and cough.
A family member quoted doctors as saying that Karunakaran is better and could be discharged either Wednesday or the following day.
The veteran leader has been out of active politics after merging his party, Democratic Indira Congress-Karunakaran -- which he launched after quitting the Congress -- with the Nationalist Congress Party.
Reports indicate that Karunakaran, who is in the winter of his long political career, is keen to return to the Congress and is awaiting signals from Delhi to make the move. He could be accommodated if there is a snap general election.