As they say in cricket, form is temporary, class is permanent (or something like that).
King would never have been a Premiership player, but he's a very, very good 1st / 2nd division player who can anticipate situations, which is one of the most important qualities in a defender, and one that hopefully will compensate somewhat for the loss in his previous level of mobility since his injury. Providing he stays injury free I agree with Gemini - one year with an option for a second.
However, I'm afraid that if he does get injured again the correct decision must be not to renew, however hard that might be. If that is the case though I hope that he'd get a benefit match at least. He has been a wonderful and loyal player.