Pablo Barrios opened the scoring inside the first minute following Alex Baena’s assist, while Johnny Cardoso had a goal disallowed before half-time.
United’s challenge weakened when Leny Yoro received a red card in the 74th minute.
Lookman, introduced for Julian Alvarez three minutes earlier, doubled Atletico’s advantage in the 79th minute. Speaking to CBS Sports Golazo afterwards, the Nigerian international praised his team's performance and described the victory as an important confidence boost ahead of their next fixture.
“We know how big of a win that is for us,” Lookman said.
“The guys put on a big performance before I come on, so it’s easy to come into the team and play.
“I tried to help the team defensively and offensively when I get the energy. It’s a big win for us; it gives us confidence going into the next game. Game after game, we have to be ready.”