Quintero made his Minnesota debut on 14 April, scoring once in a 3–2 loss to the Portland Timbers. He recorded his first assist for Minnesota on 22 April in a 3–1 loss to the Seattle Sounders. On 28 April, Quintero made his home debut for the Loons, marking the occasion by scoring once to help Minnesota get a 2–1 victory over the Houston Dynamo. After a relatively quiet three game stretch, he put his name on the scoresheet again on 20 May to give Minnesota a 1–1 draw with Sporting Kansas City. He then entered a five-game stretch that saw Quintero record zero goals or assists and Minnesota lose three MLS games and be eliminated in the US Open Cup. Quintero stepped up on July 4 and scored three chipped goals to give the Loons a 4–3 win over Toronto FC, recording the first hat-trick in Minnesota United's history. In his next 7 games, Quintero continued to stay hot, scoring 4 goals and assisting on 7 more. During that stretch he was named to the 2018 MLS All-Star game. He missed the next two games due to a calf injury. Quintero managed to record 6 assists in the final 8 games after returning from injury, but only scored once during that stretch. He had a successful debut season with the Loons, leading the team with 11 goals and 15 assists. Quintero was named to the MLS Team of the Week five times, being named MLS Player of the Week for Week 19. His 15 assists were also tied for the fourth most in the league. Despite Quintero's performances, Minnesota finished 10th place in the Western Conference, missing out on the playoffs by 13 points.