King suffered the injury moments before half-time to complete a nightmare first 45 minutes for the home side who found themsevles
2-0 down to strikes from Graham Dorrans and Chilean international Gonzalo Jara.
Bruno Berner pulled one back for Le
icester in injury-time at the end of the game.
But it wasn't enough as they dropped to sixth in the table.
And boss Nigel Pearson admitted his side had been second-best, although he was pleased with the application of his players.
"I've got no complaints with the result," he said.
"They were the better side and they passed the ball very well.
"We were disappointed that we didn't reproduce the form of our last game at QPR, but you've got to give them some credit as well.
"However, I was delighted with the players in the second half.
"I thought the work rate and commitment and effort was terrific.
"We stretched them all the way but the goal right at the end was scant consolation."
As well as King's injury, Foxes captain Matt Oakley also picked up his fifth yellow card of the campaign and will now miss Leicester's next game at home to Plymouth Argyle after the international break.
Meanwhile, Baggies boss Roberto Di Matteo was delighted with his side's display.
"We absolutely deserved to win," said the Italian.
"I think we did everything quite well.
"We defended well and we kept (Matty) Fryatt and (Martyn) Waghorn very quiet.
"We attacked well. We moved the ball around well and we controlled the game the majority of the time.
"We didn't give them any space and I thought it was a very good all-round performance."
Di Matteo also insisted that Jara, a £1.4m summer signing from Chilean club Colo Colo, is capable of playing in the Premier League after seeing him open his Albion account with a swerving 30 yard free-kick.
"Absolutely he can play at a higher level, he will be going to the World Cup with his nation next summer," enthused Di Matteo.
"I don't know what his best position is yet, maybe ask me at the end of the season."
Meanwhile former Fox Joe Mattock made his first return to the Walkers Stadium since joining West Brom in a £2m deal in the summer.
He was roundly booed by the home supporters every time he touched the ball.
But Di Matteo felt he coped well with his hostile reception.
"He responded fantastically, I thought he was very strong mentally and he had a good game," he said.
