Leicester Tigers scrum-half Ellis scored two tries for England Saxons in a 66-0 win over Portugal last Friday in his first game for six weeks and had been named in the team to face Ireland A this Friday before a call-up to cover for Care who strained ligaments in his right ankle on Tuesday.
Ellis, who wins his 23rd senior cap, will take Care's place in the starting line-up with the uncapped Ben Foden of Northampton Saints on the bench.
Paul Hodgson of London Irish will start for the Saxons at Donnybrook with Joe Simpson (London Wasps) joining the bench.
England team manager Martin Johnson said: “We all feel for Danny and wish him a speedy recovery. Harry is an experienced international and I'm sure he will do a good job for us on Saturday."
Tigers duo Julian White and Tom Croft have been named among the England replacements, while Martin Castrogiovanni starts in the Italy front row.