England examined Barry's ankle further on Wednesday and have delayed making a decision over whether the Manchester City midfielder will be available to travel to South Africa until Fabio Capello is required to name his final 23-man squad on June 1.
An unconvincing performance when beating Mexico 3-1 on Monday night has only emphasised Barry's importance to the side and Capello has underlined he is desperate to take the midfielder to the World Cup.
"Barry always played with me," Capello said. "After Owen Hargreaves it was Gareth Barry. He is one of the important players, really important."
If Barry does fail to recover from his ankle injury, then England may be short of options to fill the holding role, with Hargreaves having only played around 30 seconds of first-team action in over a year due to his long-term injury. Other candidates for the role are James Milner, to whom it is not a natural position, and Michael Carrick, who was unconvincing against Mexico.
Scott Parker may be given the chance to impress when England face Japan at the weekend but King, who is not expected to play in the friendly in Austria, has reservations about filling the role, although admits he will do so if asked by Capello.
"Do I fancy a holding midfield role on Sunday? I don't know, I am not sure about that. A lot has changed since I last played there," King said. "With not training, midfield is a different role, it is different on the body. I've got kind of used to playing at the back and adapting to certain movements.
"Midfield would be a totally different role so we will have to wait and see, one I'm willing to try if it was the case and I was asked by the manager."
One player unlikely to play in the withdrawn midfield role is Steven Gerrard. Capello feels the Liverpool captain is more effective in an advanced position and will not risk reducing the potency of one of his most talented players.
"Steven is really dangerous when he gets close to the box," Capello said. "If he has to play in the same position as the other midfielders he is good, but he is too far from the box. He has to get between two lines, the back four and the midfield. That is where you go to shoot.
"I am lucky because his performance level is always high, but while I like to go forward you always need a holding player in front of the back four."