The 14-goal striker, 32, has only managed to feature in one of Blues' last four matches due to a troublesome knee injury, while he has only found the net once, away at Burnley, since the end of November.
City boss Lee Clark has revealed the player will now undergo surgery in a bid to resolve the problem ahead of next term.
"Marlon has an operation today, so he will be out for the season," Clark told City's official website ahead of Tuesday night's clash at home to Blackpool.
"He's basically going in to see what they find. There is obviously an issue in that it keeps swelling up. They're cleaning up the cartilage bit and we're hopeful that's the minimum that's needed.
"They don't want to go down the Steven Carr and David Murphy route just yet (in terms of major knee surgery) and Marlon is keen not to do that. We'll know leading up to pre-season when he starts his rehab, before the rest of the group come in with the physios, if it's been successful or not.
"If it hasn't been, the worst-case scenario is that he misses all of next season as well. Fingers crossed that this operation proves successful."
Clark will not be bringing in a replacement for King, with City's goalscoring exploits now resting largely on the likes of Nikola Zigic, Peter Lovenkrands and Wes Thomas over the remaining 11 games of the season.
"Of course it is a massive blow," Clark added. "As well as losing goals, we'll also lose his leadership in the dressing room etc.
"But it's a chance for somebody else now. We've got three strikers and it's up to them now to get us the goals and play themselves into the team. There also might be a young striker who comes to the fore in the next couple of weeks. Akwasi (Asante) is starting to get fit. Young Reece Hales always impresses me when I watch him in the youth team and development squad and Koby (Arthur) can play up there."
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