King returned to City's starting line-up for the 0-0 draw with Sheffield Wednesday on 19 February but was replaced by Peter Lovenkrands in the 83rd minute.
The 32-year-old has not featured since and Birmingham manager Lee Clark has confirmed he will miss the rest of the current campaign.
City lost 5-2 away to Hull on Saturday and Clark has also challenged his fringe players, including his forwards, to fight for a place in the team.
"It's a blow for Marlon and a blow for us," Clark told Sky Sports News.
"He is going to go under the knife today (Monday) and get the knee checked out, so we can get his rehab sorted for pre-season and hopefully he should be OK.
"For other players now, it's an opportunity for them with the news of Marlon, and all over the team, because there are places up for grabs after you lose a game like that.
"The lads who have maybe not been involved have got an opportunity now."