Strettle missed the first Test defeat by South Africa in Durban last weekend with a calf injury but had been included in the side to face the composite team in Kimberley.
It was thought the game at GWK Park might be an opportunity for the Saracens back to prove his fitness, but a team note from the Rugby Football Union suggested he was already in good shape.
It read: "Strettle has been removed from the line-up to offer selection options ahead of Saturday's second Test against South Africa."
England must make two changes in their back line for the second Test in Johannesburg after losing full-back Mike Brown and centre Brad Barritt to injury.
A vacancy could arise on the wing if head coach Stuart Lancaster decides to switch Ben Foden from out wide to his preferred full-back position.
Ugo Monye offers another wing option while Alex Goode, who was due to face the Southern Barbarians, could be an alternative full-back.
Centre Jonathan Joseph could earn a first start in Barritt's place, although he could play outside centre if Manu Tuilagi moves inside, or even to the wing.
There are other permutations depending on whether Owen Farrell is moved from fly-half.
Lancaster is due to announce his team on Thursday.
George Lowe started in Strettle's place against the Southern Barbarians with Phil Dowson added to the bench.
All members of the England touring party made the one-hour flight from Johannesburg to watch the tour match in the diamond mining town of Kimberley.
Prior to the match, the members of the Test side not playing all participated in a coaching clinic with around 200 children from local schools, none of whom had played rugby before.
Winger Chris Ashton said: "I have come here today to watch the lads play later and do a bit of coaching.
"You don't get many better things out of rugby than watching kids like this learning and taking the first steps playing rugby."
Ashton felt the clinic was a nice distraction from the pressures of the Test series.
The 25-year-old said: "It was a pretty tough test. Unfortunately we didn't win - the third quarter let us down and that is something we need to concentrate on.
"But it is a day off today, so just to get out of the hotel and do something different is great."
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