After suffering a shoulder injury in training on Thursday Hewitt had the hugely disappointing news that he would be unavailable for any part in the three-test tour. against the Springboks and Pumas.
Ashton had been called up in place of Josh Adams, after the Worcester wing was precluded due to Premier Rugby release restrictions. Scarlets Flanker Aaron Shingler will also miss the trip due to injury in the Pro14 final yesterday.
The Dragons contingent for the tour includes co-captain Cory Hill plus Ellliot Dee, Hallam Amos, Aaron Wainwright and Ross Moriarty.
