Lee Kang-in made his debut in front of his home fans wearing the Atletico Madrid (AT Madrid) jersey. Lee came on as a substitute in a friendly match against Manchester City (England) in the 2026 Coupang Play Series held at Seoul World Cup Stadium on the 9th, playing approximately 30 minutes on the pitch.

Having transferred from Paris Saint-Germain (PSG) to AT Madrid, Lee signaled a new beginning by wearing the number 7, the shirt number of Antoine Griezmann. A crowd of 50,078 spectators packed the stadium, greeting Lee's new challenge with enthusiastic cheers. Lee, who started the match on the bench, came on in the 18th minute of the second half in place of Rodrigo Mendoza, warming up amid a fervent atmosphere as the crowd chanted his name.

Immediately after coming on, Lee aimed directly at goal with his signature left-footed free kick, but the shot unfortunately missed the target. He then actively participated in the build-up play, moving between the left and right channels behind the frontline striker. In the 26th minute of the second half, he played an accurate pass from the center to the flank to change the direction of the attack, and in the 30th minute, he connected a cutback pass directly into a left-footed shot, but the ball went over the crossbar, squandering the chance for a debut goal. Although he did not register an attacking point, he showcased his exceptional left foot, passing, and aggressive movement, opening the curtain on his bid for a starting spot in the new season.

The match ended with AT Madrid falling to Man City 1-3. Jorge Dominguez scored the opening goal from a corner kick situation in the 43rd minute of the first half, but Man City turned the game around in the second half with consecutive goals from Antoine Semenyo and Omar Marmoush, followed by a clinching goal from Rayan Ait-Nouri. After the final whistle, Lee greeted the crowd, officially marking his new beginning at AT Madrid.