The youngster landed awkwardly on his knee following a challenge by Ecuador midfielder Carlos Gruezo and is now a doubt for England's three group games at the World Cup
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The 20-year-old was scheduled to undergo a scan following the game to assess the damage, but the Football Association have not confirmed an injury timescale for the player.
"Alex Oxlade-Chamberlain sustained a medial knee ligament injury during the World Cup warm-up game against Ecuador in Miami on Wednesday," an official Football Association statement read.
"The England medical team are continuing to monitor and treat the injury."
The former Southampton winger took to Twitter after his diagnosis, and revealed he remains positive about recovering for the tournament in Brazil.
"Gutted to have picked up an injury yesterday. I'm staying positive and getting to work with the medical team on recovery..Fingers crossed!" he wrote.
England begin their Group D campaign against Italy on June 14 before taking on Uruguay and Costa Rica in their remaining first round fixtures.