"We are fortunate in that Rhys knows this squad well, it's a great chance for him and he will take little time to settle in," said forwards coach Robin McBryde.
"With only two hookers on the tour with us, he will definitely play a part for us on Saturday which won't be easy for him after joining us so late, but we have confidence in him."
Llanelli Scarlets hooker Rees sustained a tear to his calf muscle int the 43-17 defeat by the Springboks last Saturday, and will now be several weeks in rehabilitation.
"It's a big disappointment for Matthew especially as he has become such a major influence on the team. He is very sad to be missing out on the second Test but we know he'll be putting a tremendous effort in to be back on the field as soon as possible."
Thomas Rhys THOMAS (CARDIFF BLUES - Cap No 1,035)
DoB: 23.04.82. 6'0" / 1.83m. 16st 8lbs / 105kgs
International record: 26 caps (W 10, L 15, D 1; 5 pts - 1T)