Easter pulled out of England's 41-10 victory over Georgia and Thomas Waldrom has been called to New Zealand as "temporary cover".
Easter took the daredevil plunge over the Nevis River in Queenstown last week and there was speculation he may have ricked his back in the process.
But England have confirmed the Harlequins number eight trained through last week and it was only on Saturday, the day before the Georgia game, that he began to feel a tightness developing.
The England medics decided not to take any risks and pulled him out of the match, which James Haskell started in his place.
Waldrom is on his way to Dunedin, although he will not be able to link up with the squad in any official capacity unless he becomes a permanent replacement.
England are also monitoring prop Matt Stevens, who suffered a sprained ankle against Georgia but is said to be improving.
Alex Corbisiero (calf) and Ben Foden (side) are not major injury concerns and Chris Ashton took a full part in training after hurting his elbow in the act of scoring his second try on Sunday.
England have still not replaced Andrew Sheridan in their squad after the Sale prop was ruled out of the tournament with a shoulder injury.
Sheridan does not have to be replaced with another prop and England have decided to keep their options open.
At this stage, Waldrom is en route to his native New Zealand simply to provide cover for Easter.
In 2003, England called up Austin Healey and Martyn Wood as injury cover in the same way as Waldrom has been now and both ended up flying home again.
Waldrom was caught off guard by the call-up.
He arrived at Leicester's Oval Park training base on Monday expecting to take part in a review session of their one-point Premiership win over Newcastle.
But instead he was told by Tigers director of rugby Richard Cockerill to pack his things for New Zealand.
"It was a swell feeling and I had to rush home and tell my wife what was happening," Waldrom told the Leicester Mercury.
"I am going out as cover so it's difficult to know what might happen. We will just have to wait and see."
Waldrom was a member of England's initial 45-man World Cup training squad but he was hampered by a calf injury and was cut before the August warm-up internationals.
The 28-year-old has played in all three of Leicester's Aviva Premiership games and scored a try in the opening-day defeat to Exeter Chiefs.
Waldrom was born in Lower Hutt, near Wellington, and only discovered in March that he qualified for England courtesy of his grandmother.
Leicester had signed Waldrom from the Crusaders on the basis that he would not be called up for international duty.
His departure weakens Cockerill's back row resources even further. Jordan Crane and Craig Newby are both injured while Tom Croft is in the England squad.
"I am pleased for Thomas but it is far from ideal for us," said Cockerill.
"We recruited Thomas because we thought he would have to wait three years before he qualified for England - now this happens.
"We have just got to crack on and get on with it."
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