Blues' top scorer has had surgery on his left knee, meaning he is out until the summer.
"It's massively disappointing to miss the remainder of the season," he told the Birmingham Mail.
"I hoped I would be able to see it through, but in the end the operation was needed.
"I will take six to 12 weeks to recover and without a doubt I will be ready and flying for when we report back for pre-season.
"The surgeon was happy with the way the operation went, and I have already started my rehabilitation, doing some static exercise.
"It was a cartilage problem. Some of it, a floating body, had lodged under my kneecap and it was restricting me.
"I wasn't playing at 100 per cent and in the Sheffield Wednesday game (on February 19) I struggled through for 80-odd minutes. There was no 'give', it was as if I was playing on one leg.
"So it's been cleaned up and all is well, the operation has gone to plan and now it's about the recovery."
The 32-year-old almost left the club earlier this year, when Birmingham were listening to offers for their biggest earners.
Shanghai Shenhua tried to take King to China before their transfer window closed, but the moved failed to materialise.
"It was out of my hands. Nobody was aware of what was likely to happen, what the club needed to do," he said.
"And as far as I was concerned, I didn't want to leave anyway. I just wanted to concentrate on playing for Birmingham City."