Simpson is among five changes to the Force's starting XV for Sunday's clash with the Highlanders in Invercargill.
Captain Michael Wells and lock Jeremy Williams return from head knocks, while Zach Kibirige (hamstring) and Ryan McCauley are also included.
Former All Blacks lock Jeremy Thrush will miss at least two weeks due to a heel injury.
The biggest selection surprise was Simpson, who was not even in the Force's original team roster for this season.
An ACL tear for new signing Henry Robertson forced coach Simon Cron to scramble for another scrumhalf option, and Simpson - a South African-born Englishman - was one of the players recommended to him.
Simpson previously played for the Worcester Warriors and in February signed a two-year deal with Saracens.
He will play out the Super season on loan with the Force before returning to England to link back up with Saracens.
"When we lost Robbo (to injury) in Japan, we were looking for someone who can manage games and kick well," Cron told reporters on Thursday.
"We did a heap of homework on (Simpson) with footage and tracked him.
"I Zoomed him Christmas Eve, I'm sure he enjoyed that, and had a chat for probably an hour, just interviewing him and questioning him.
"From that Zoom call, I could tell he was the right person for our team."
Cron is hopeful hooker Folau Fainga'a will be able to play against the Highlanders.
Fainga'a was subbed out of last week's 21-18 win over Moana Pasifika with a head knock.
Normally, players would miss 12 days under the league's concussion protocols, but there can be exceptions.
"After the game he sat some more testings, more HIAs, passed all of them, passed the next day," Cron said.
"He had no symptoms. So he's still going through a process where they can apply for him to play off the back of the fact that he hasn't had head knocks before.
"He's really confident, but it has to go through medical."
Thrush was brought out of retirement on the eve of the season, and is set to decide whether or not he will play again once he has recovered from his heel injury.
"We'll always try to make room for him, but that's going to be his choice," Cron said.
The Highlanders have lost their opening three matches this season, while the Force are 2-1.
FORCE SQUAD: Tom Robertson, Folau Fainga'a, Santiago Medrano, Jeremy Williams, Ryan McCauley, Michael Wells, Ollie Callan, Rahboni Warren-Vosayaco, Gareth Simpson, Bryce Hegarty, Toni Pulu, Hamish Stewart, Bayley Kuenzle, Zach Kibirige, Chase Tiatia. Res: Feleti Kaitu'u or Tom Horton, Angus Wagner, Siosifa Amone, Felix Kalapu, Tim Anstee, Issak Fines-Leleiwasa, George Poolman, Manasa Mataele.
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