Pat sustained a knee injury early in his debut for Scotland in their defeat to Samoa on Saturday.
Fraser signed a one-year deal with Glasgow last month, after arriving at Scotstoun from Heriotís earlier in the season to provide cover for injuries and international call-ups.
The 23-year-old uncapped hooker played for the Scotland Club International team earlier in the year and made four appearances for the Warriors in the RaboDirect PRO12 last season, including a substitute appearance in the play-off defeat to Leinster.
Speaking to glasgowwarriors.org Fraser said: "It's a bit unreal to be honest, a few months ago I was playing amateur Premier Division rugby and now I've been called up to the Scotland squad.
"I'm immensely proud and hugely excited. I've had one day of pre-season training as a professional and now I'm going out to South Africa.
"It's hugely unlucky for Pat (MacArthur). He was unlucky not to get on during the 6 Nations and I know how much that drove him on until the end of the season.
"I've got family out in South Africa so I've been out there before, it's an amazing place, and it'll be a good experience.
"It'll give me a boost in confidence going out and training with the boys and hopefully when I get back I'll be raring to go for Glasgow next season."