With qualification for the last-16 already secured, Joao Cancelo was the only City star to start, with both Ruben Dias and Bernardo Silva on the bench.
And top spot in Group H was confirmed ahead of second-placed South Korea despite Hwang Hee-chan’s dramatic injury-time winner at the Education City Stadium in Qatar.
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Portugal had won their first two matches against Ghana and Uruguay and took the lead in their final group game inside five minutes when Ricardo Horta swept in Diogo Dalot’s low cross.
But South Korea levelled midway through the first half when a corner rebounded off Cristiano Ronaldo’s back into the path of Kim Young-Gwon who turned in the loose ball.
With Uruguay beating Ghana 2-0 in the other game, South Korea needed a second to snatch an unlikely passage to the last-16.
Portugal made a raft of changes as the game appeared to be petering out with Bernardo coming on for the final 10 minutes.
But in the 91st-minute, Son Heung-Min raced clear to prod a pass through to Hwang to score a late winner and send them through with Portugal.
