"We have our fears there might be some ligament damage and a scan is required," England manager Roy Hodgson told a news conference at the Sun Life stadium.
Oxlade-Chamberlain hobbled off just past the hour mark after Ecuador's Carlos Gruezo rolled on to his knee having kicked the back of the Arsenal player's right foot in a poor challenge.
"His knee buckled under the challenge as the player turned into him," Hodgson added.
"He'll have a scan tomorrow, I'd imagine. When we've had the scan, we'll know more."
The 20-year-old, playing his first match since late April due to a groin problem, was one of England's most dangerous players in their penultimate friendly before the World Cup.
(Reporting by Ken Ferris, editing by Ed Osmond)
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