The Harlequins' wing was set to fly home on Tuesday night after being laid low for several days by a stomach virus that forced him out of last Saturday's first Test in Bloemfontein.
Strettle spent several days in hospital for rehydration treatment and was discharged over the weekend, but he was not fit enough to be in contention for the second Springboks clash this weekend.
Gloucester pair Iain Balshaw (shin) and Nick Wood (torn chest muscle) have already returned home.
England captain Jason Robinson, meanwhile, says he will be fit for the weekend despite missing training in Centurion on Tuesday.
Robinson took a bang on his leg during the 58-10 first Test loss, but he said: "I am not concerned."
Jonny Wilkinson, who was also in the wars at Vodacom Park, trained, as did Andy Farrell and Peter Richards, who missed the Bloemfontein clash after picking up the stomach virus that swept through the England camp last week.
Robinson was joined on the sidelines by Andy Hazell (knee) and James Simpson-Daniel, who was feeling under the weather.