The 20-year-old attacker is close to joining Porto in a £10m deal. Quintero was one of Sir Alex Ferguson's final transfer targets, and he instructed club scouts to keep an eye on the youngster through much of last season.
United officials watched Quintero dominate the South American under-20 championships two months ago - the midfielder finished the competition as second top scorer with five goals.
The left-sided playmaker has been superb at the under-20 World Cup in Turkey, and Porto have moved quickly to capitalise on the transition at United and clinch a deal for Quintero.
New United boss David Moyes has pressing priorities elsewhere, with a change in back-room staff and Wayne Rooney's future at the top of his list.
That's allowed Porto to nip in, with club representatives flying to Italy this weekend to seal the £10m capture.
Quintero featured heavily for Italian side Pescara last season, and was signed on a permanent deal in January after initially joining on loan from Atletico Nacional.
But Pescara's relegation to Serie 'B' has brought interest in the player, with Inter Milan, Juventus, Porto and United linked.
Porto have stolen a march though, following the £38.5m sale of Colombian international James Rodriguez. Quintero has been identified as the man to replace Rodriguez, and the Portuguese champions have accelerated the deal to beat interest from United.