Strettle, who spent several days in hospital last week for rehydration treatment after being laid low by the stomach virus that swept through England's camp, follows Gloucester pair Iain Balshaw (shin) and Nick Wood (torn chest muscle) in making an early exit.
Although the virus appears to be showing signs of easing, England head coach Brian Ashton will not announce his starting line-up until Wednesday afternoon at the earliest.
On the plus side, fly-half Jonny Wilkinson trained on Tuesday 72 hours after leaving first-Test venue Bloemfontein with a cut face and bruised lower back, while centre Andy Farrell and scrum-half Peter Richards - both virus victims - also featured.
But Robinson (knee fluid), flanker Andy Hazell (bruised knee) and wing James Simpson-Daniel (ill) took no part in the session.
Given England's continuing state of uncertainty, many pundits believe the Springboks will better Saturday's record 58-10 victory at Vodacom Park, when they scored seven tries and left the world champions reeling from a second-heaviest loss in their 136-year Test history.
Robinson though, who moved to allay any fears about his fitness, claimed England could not wait for another appointment with the Springboks.
He said: "We are looking forward so much to going out there on Saturday. There is a lot of frustration after the Bloemfontein game, but Saturday cannot come around quickly enough, to be honest.
"We know we can play a lot better than that, but you cannot give South Africa turnover ball like we did or they will punish you.
"We've got a team that believes it can win on Saturday. Yes, we were beaten by a heavy margin last weekend, but we've looked at the game and, if we sort it out, there is no reason why we can't.
"We truly believe we can turn things round. At the end of the day, we will see what happens on Saturday - the proof is in the pudding."