“We’re all gutted to lose Jac – he’s a world-class player, as well as a great man off the pitch,” said Tandy.
“This gives an opportunity now for someone else and Harri has been performing strongly for the Ospreys for a consistent period.”
Tandy will be forced into at least one back-row change when he names his squad for Saturday’s Test against Japan.
Alex Mann and Aaron Wainwright started in the back row alongside Morgan against the Pumas with debutant Olly Cracknell coming on along with lock or flanker Freddie Thomas.
Uncapped Ospreys number eight Morgan Morse and Scarlets’ Taine Plumtree are also in the squad.
Deaves, 24, is the only specialist open-side after Tandy opted to overlook Leicester flanker Tommy Reffell, who would not have been available for the Springboks finale that is outside World Rugby’s official Test window.
Mann, a mobile blind-side by trade, had been mooted as the cover for the seven jersey before Morgan’s injury blow.
