Oxlade-Chamberlain fell awkwardly after a clumsy challenge by Carlos Gruezo in England's 2-2 draw against Ecuador at Sun Life Stadium in Miami on Wednesday night. Scans taken yesterday showed the extent of the damage, which looks not to be as bad as first feared.
"Alex Oxlade-Chamberlain suffered a medial knee ligament injury during the game against Ecuador," an FA statement read. "The England medical team are continuing to monitor and treat the injury."
Reports suggest Oxlade-Chamberlain has only suffered a ligament strain, rather than a tear. If that is the case then he can expect to be missing for two to three weeks.
That would rule the midfielder out of England's first Group D match against Italy on June 14, but he could make the second fixture against Uruguay in Sao Paulo five days later.
Roy Hodgson will not make a snap judgement on whether he wants to replace Oxlade-Chamberlain.
The England manager is willing to give his playmaker time to prove his fitness and see how he reacts to his rehabilitation in Miami, where England take on Honduras in their final warm-up game tomorrow.
Also, under FIFA rules, Hodgson has until June 13 - 24 hours before England play Italy - to make any changes to his squad.
Oxlade-Chamberlain joined his team-mates for a recovery session at the squad's base in Barry University in Florida yesterday, although he was wearing a leg brace. The former Southampton midfielder was able to walk relatively freely on his right leg, which buckled under the pressure of Gruezo's foul in the 63rd minute.
Before the injury, Oxlade-Chamberlain looked to have greatly improved his chances of lining up against the Italians in Manaus on June 14. Starting his first game since April 20, Oxlade-Chamberlain made a series of surging runs and clever passes in the sticky Florida heat.
After the match, Hodgson said he hoped the player would be fit to board the plane to Brazil tomorrow night.
The England manager said: "He was excellent. He was back really to his best so all I can do now I suppose is cross my fingers and touch every piece of wood I pass by that it is not serious."
Hodgson does not know whether Gruezo aimed to hurt Oxlade-Chamberlain in retaliation for a kick on the Ecuador player by the Arsenal man moments earlier.
"He [Gruezo] stumbles into [Oxlade-Chamberlain] but whether he meant it or not only the Ecuadorian knows," Hodgson added.
Hodgson has several options at his disposal should he decide drop Oxlade-Chamberlain. Manchester United midfielders Michael Carrick and Tom Cleverley are on England's standby list, but they have not spent a minute with the squad this summer.
Carrick has been on holiday in Ibiza while Cleverley was pictured at a racecourse with his girlfriend on Wednesday. Unlike Carrick and Cleverley, John Stones and Jon Flanagan have been present ever since Hodgson announced his World Cup squad.
The defenders were brought along to Portugal and Miami as cover for Phil Jones, who suffered a shoulder injury in the final week of the domestic season. Both players can operate at right-back, which would allow James Milner, who filled in there on Wednesday, to concentrate on playing in his normal midfield role.
Stones, who earned his first cap against Peru, and Flanagan, who made his debut off the bench against the Ecuadorians, are untested but full of future promise.
Oxlade-Chamberlain, though, is staying optimistic. He tweeted: "Gutted to have picked up an injury yesterday. I'm staying positive and getting to work with the medical team on recovery..Fingers crossed!"