The English full-back, making his Champions League debut in the white jersey under Xabi Alonso, had barely settled on the pitch before being forced to leave after feeling sudden pain.
As he sprinted forward with the ball, Alexander-Arnold suddenly clutched his left hamstring, the pained expression on his face revealing the latest setback in a challenging start to his Real Madrid career since leaving Merseyside.
Alonso had aimed to rotate his full-backs this season, giving both Carvajal and Alexander-Arnold two La Liga starts each in the opening four matches.
The idea was to keep players fresh, maintain healthy competition, and ensure depth across all competitions. But with Alexander-Arnold now sidelined, that plan faces disruption.
Carvajal, a club stalwart and academy graduate, once again demonstrated why claiming the position from him is a daunting task, solidifying his hold on the starting spot.