The 24-year-old earned his first of two caps off the bench on Scotland's tour to South Africa last year, but in the second of those games - against the Springboks - he suffered a serious knee injury that kept him out of rugby for all but the last two months of the season.
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Horne made a satisfying return to action, playing six times for the Warriors in their run to the RaboDirect PRO12 final, and was initially called up for the second part of Scotland's summer tour, which will take them to Argentina and South Africa. However, he was brought out early after Duncan Taylor suffered a calf injury in the victory against the USA.
The other injury-enforced change sees Edinburgh lock Grant Gilchrist take over, as he did in the course of the USA match, from Jim Hamilton, who damaged an ankle. The tactical changes have flanker Kelly Brown coming in for Blair Cowan, who drops to the bench, and tighthead prop Moray Low taking the shirt of Geoff Cross, who also has the consolation of being named as a replacement.
While head coach Vern Cotter had Mark Bennett and Max Evans available to cover the loss of Taylor, he explained that he preferred the playmaking instincts of Horne in the inside-centre position.
Cotter said: "Ten and 12 are the people who have to make decisions on the field, and he [Horne] is adept at that. If we are losing shape he can offer it, and I think that's important. I really want to develop 10 and 12 as decision-makers and playmakers."
Cotter's thinking is in line with his New Zealand roots, where the 10/12 combination of first and second five-eighth puts a lot more emphasis on the inside-centre's responsibility to make things happen.
Intriguingly, Finn Russell, who retains his place at fly-half after an impressive display against the Eagles, has played any games at 12, while Horne has also played at 10.
"We work well together," said Horne. "A lot of the time we'll just have a little chat and come up with the starter play we want to run and he kind of takes control of that first and second phase and after that I become his eyes.
"Finn has so much to focus on. He's looking at the forwards, concentrating on the breakdown, so it's a lot easier for me to have my head up and I can tell him if there is space in behind. I normally just call a play and he can go through with that, he doesn't need to worry if boys are in the right position. It works pretty well I think. It's good.
"He's brilliant to play with, he's so cool. He's got a great pass off both hands and will always find you so I am really looking forward to it."
Horne admitted that he had been on a steep learning curve since he arrived in Toronto on Monday.
"When I got here he [Cotter] told me I had a lot of work to do," Horne said. "I was up to God knows what time learning all the calls.
"I had quite a lot to pick up. On Tuesday morning I got the heads up to say I was playing. I did not want to come in and let anybody down. Training had been very good and I wanted to make sure everything I did was up to that standard."
Brown came to North America after playing in Heineken Cup and Aviva Premiership finals with Saracens on successive weekends, and was understandably rested for last weekend's match in Houston.
It had been expected he would resume the captaincy on his return, but that role has been retained by Greig Laidlaw, who deputised impressively against the Eagles.
Cotter also signalled a change in thinking away from the traditional open-and-blind flank combination, preferring the kind of left-right arrangement he was more familiar with while coaching in France.
Gilchrist will call the lineouts, but Cotter indicated that he wants more players to be able to take on the duties in the future.
"Richie Gray has had a piece of paper with him this week and he is practising," the coach explained. "Kieran Low has been practising as well. We need someone else as backup if Grant leaves the field. Those things are important within the group - we want to share the responsibility."
Cotter encountered Canada coach Kieran Crowley, the former All Black full-back, on the playing field when he was at Bay of Plenty and Crowley was at Taranaki.
With that shared hinterland, he explained that he expects the Canadians to play with ball in hand as much as they can.
Crowley also named his team yesterday, making three changes to the side that lost 25-34 to Japan in Vancouver last weekend. The most significant of the new arrivals is lock Jamie Cudmore, who played under Cotter at Clermont Auvergne.
Glasgow's DTH Van der Merwe, who has only just come back from a shoulder injury, is on the bench, while former Warriors winger Taylor Paris has been chosen on the wing.