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 last night.
Scans taken earlier today 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 out for two to three weeks.
That would rule Oxlade-Chamberlain 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 Oxlade-Chamberlain time to prove his fitness and see how he reacts to his rehabilitation programme here in Miami, where the Three Lions play Honduras in their final warm-up game tomorrow.
Plus, 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 earlier today, although he was wearing a leg brace.
The former Saints 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 on Saturday 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.”