But British media reported Itoje was performing a forwards' drill when a side-step caused him to fall to the ground in pain and he hobbled out of the session with a suspected knee injury.
"Maro has had a minor tweak and the medical staff are taking a look at it," England attack coach Scott Wisemantel said.
"He's being managed day by day anyway, I don't think it's anything serious. We've just got to wait and see what the medics say. We'll see how he pulls up tomorrow."
Itoje has not played since he was withdrawn during England's 32-20 win over holders Ireland in their opening game last month.
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The call-up of the 23-year-old Ewels was announced in a statement on the England rugby website (www.englandrugby.com) which also noted that Gloucester's Ollie Thornley, who can play on the wing or at centre, had been retained as cover for Jack Nowell.
England are second in the Championship, two points behind Wales who beat Jones's side 21-13 in Cardiff last month.
(This story corrects headline to say "as cover for", not "to replace", as Itoje remains in the squad.)
(Reporting by Rohith Nair in Bengaluru, editing by Ed Osmond and Hugh Lawson)
