The 30-year-old England international played fewer than 10 league games last season due to his chronic knee problems which have dogged his career.
And Redknapp insists he will not be able to hand King a new contract unless he is playing more regularly.
"Ledley's a difficult one," said Redknapp.
"He has to play this year. I can't go to the chairman and say, 'Give Ledley a contract', if he has only played two games because he isn't going to do that.
"This is Ledley's big year, he's got to play 20 games or so, really. Ledley needs to put some games in this year to get another contract."