The 20-year-old Arsenal man looked lively during the 2-2 draw in Miami but then had to be withdrawn just after the hour mark when a tackle from Carlos Gruezo left him limping.
England begin their World Cup campaign against Italy on June 14 so it looks like Oxlade-Chamberlain will definitely miss the Manaus clash and the next match is against Uruguay on the 19th.
Sky Sports reporter Nick Collins said: "No official word yet but our sources say Alex Oxlade-Chamberlain is likely to be out for between 10 and 21 days.
"If it is only the 10 days, he might be available for the game with Uruguay and, of course, the later Group match against Costa Rica.
"However, if he is out for the full three weeks, that would rule him out of all the Group games and that would probably mean the end of Alex Oxlade-Chamberlain's World Cup hopes.
"Either way, it looks like manager Roy Hodgson has a decision to make.
"It would be a shame if Oxlade-Chamberlain missed out. He showed he has a lot to offer England with his pace and ability to beat players.
"It may not be as bad an injury as first feared in terms of possible ligament damage, so England will hope he can stay with the squad, but it is a case of wait and see."
Regarding the possibility of England calling up a replacement for Oxlade-Chamberlain, Collins added: "(Defenders) Jon Flanagan and John Stones are with the squad at the moment, so Roy Hodgson may decide to keep one of them on.
"However, neither would be a like-for-like replacement, so he may call up somebody completely different."
Hodgson said of Oxlade-Chamberlain after the Ecuador game: "It would be an enormous blow to lose him. He did ever so well and looked really, really sharp.
"His knee buckled under the challenge, as the player turned into him. At the moment, he doesn't know how to feel, because he doesn't know how serious the injury is."