The 20-year-old made his international debut in Friday's Euro 2016 qualifying win over Armenia, and after coming on as a second-half substitute, he was left all alone to head fellow replacement Ricardo Quaresma's cross into the net and ensure Portugal finished 2014 with a third consecutive 1-0 victory.
It was another telling contribution from Quaresma, who had come off the bench to have a hand in the goals that sealed Portugal's two previous wins.
The fixture had been billed as a clash between the two men widely considered to be the greatest players in the world, but neither Lionel Messi nor Cristiano Ronaldo produced any true moments of magic to spark the match into life.
Messi came closest to scoring when he struck the base of a post inside the opening quarter of an hour before both he and Ronaldo were taken off at half-time.
Argentina had the better of the chances throughout, but the World Cup runners-up could not come up with a decisive moment to change the course of the game.
Messi and Angel di Maria - playing on his home ground - were two of just four men to keep their place from Argentina's 2-1 victory over Croatia, in which Messi had netted the winner from the penalty spot.
Portugal handed an international debut to Tiago Gomes as Fernando Santos made four changes from their victory over Armenia.
Messi was involved in the first chance of the game in the fifth minute as he slipped a pass to Di Maria, who curled a low shot just wide from the edge of the area.
Argentina looked the more threatening in the early exchanges, and Messi struck the base of Beto's left-hand post from the narrowest of angles in the 11th minute.
Ronaldo had his first sight of goal 18 minutes later but, after twisting and turning inside the box, he blazed a left-footed shot high and wide.
Messi fizzed a 20-yard free-kick narrowly over the crossbar, before Ronaldo was forced to apologise after inadvertently kicking Lucas Biglia in the stomach.
With the two star attractions withdrawn at the break, Di Maria attempted to take centre stage with a thumping drive from outside the area that was comfortably held by Beto.
Messi's replacement Nicolas Gaitan headed wide from Javier Mascherano's lofted pass in the 64th minute as Argentina continued to look the more likely to break the deadlock.
Yet it was Portugal who ultimately prevailed courtesy of an injury-time winner.
Adrien Silva's shot deflected off Eder into the path of Quaresma, whose right-wing cross was met by a diving Guerreiro from six yards.
