"The England medical team are continuing to monitor and treat the injury," the English FA said in a statement on its website.
British media reported that Oxlade-Chamberlain could be out for between 10 days and three weeks, raising question marks over whether he can be kept in the squad, with manager Roy Hodgson able to call up a replacement if necessary.
Oxlade-Chamberlain looked dangerous in the 2-2 draw before hobbling off.
The 20-year-old, playing his first game since April 20 following a groin injury, will almost certainly miss England's opening Group D game against Italy in Manaus on June 14 but could be fit for the second match against Uruguay five days later.
If he is out for three weeks, though, his World Cup dream is likely to be over, but Hodgson is able to call on Michael Carrick or Manchester United teammate Tom Cleverley from the standby list.
Oxlade-Chamberlain was one of England's best players before an awkward tangle with Carlos Gruezo after 63 minutes forced him to limp off at the Sun Life Stadium where 21,534 fans, mostly wearing the colours of fellow World Cup combatants Ecuador, had turned out on a muggy afternoon.
Hodgson was also encouraged by the performance of another young midfielder, Ross Barkley, who displayed some silky skills and set up Rickie Lambert's goal that put England 2-1 ahead before Ecuador substitute Michael Arroyo's cracking equaliser.
ENORMOUS OBSESSION
Barkley sparkled throughout with his willingness to run at defenders, reviving memories of former England great Paul Gascoigne, but Hodgson bristled at the post-match news conference when asked about the young Everton player.
"Whenever I come to a press conference there always seems to be an enormous obsession with one player," he said. "I will obviously be prepared ... to start any of the players in this tournament.
"But I don't know I am prepared at the moment to share your obsession with Ross Barkley's performance today.
"He lost the ball an awful lot of times as well and I think if he is going to really be the player that we want him to be he has got to make better decisions about when he turns with the ball, when he keeps the ball," said Hodgson.
"Having said that, that's not a severe criticism of him, but I have to say I thought there were other performances today that certainly would have merited more attention than Ross Barkley's - so maybe that is where the surprise comes from.
"Oxlade-Chamberlain was outstanding, Phil Jones was outstanding, Jack Wilshere was very, very good in the middle of the park, Lambert scored a magnificent goal - there are four off the top of my head," added the England manager.
Hodgson reacted similarly at a previous news conference when asked about Rooney who was deployed in an unfamiliar role on the left wing against Ecuador and failed to shine although he did grab the equaliser on the half-hour mark.
"I thought Wayne Rooney played very, very well..." Hodgson said, having previously criticised the media for what he also described as their "obsession" with the Manchester United striker and his role in the England team.
Rooney's goal cancelled out the opener from Enner Valencia, whose namesake and team captain Antonio, a club mate of Rooney's, found himself red-carded along with England substitute Raheem Sterling for a fracas late in the game.
Fortunately, neither player will be banned for any matches at the tournament in Brazil, although Sterling is suspended for England's final warm-up game against fellow World Cup combatants Honduras back at the Miami Dolphins stadium on Saturday where Hodgson will field a much stronger side.
