Wing Hewitt damaged his shoulder in training and will miss the clash with the Springboks in Washington on June 2, as well as the two-match series versus the Pumas later the same month.
However, head coach Warren Gatland has opted against naming a replacement for Hewitt.
A Welsh Rugby Union statement said: “Scarlets flanker Aaron Shingler has been ruled out of Wales’ summer tour. Shingler suffered an injury in the Pro 14 final yesterday that requires further assessment and precludes him from taking part in the tour.
“Cardiff Blues’ Josh Turnbull has been called into the squad and will link up with them immediately.
“Ashton Hewitt has also been ruled out of the tour and released from the squad.
“Hewitt suffered a shoulder injury in training on Thursday that also precludes him taking any part in the three-Test tour.”
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