The obvious choice to replace Alexander-Arnold on the right flank is Dani Carvajal. The veteran right back has already played 276 minutes in 2025–26, showing no signs of the ACL injury that kept him sidelined for almost all of last season.
Carvajal cannot fill in for Alexander-Arnold in every single match for the next six to eight weeks, though, especially in the Champions League where he faces up to a three-match ban for the red card he received against Marseille.
Alonso will then have to look to David Alaba or Raúl Asencio to round out his backline, though both players have struggled at the position in the past. Fede Valverde is also an option should Alonso trust the natural midfielder at the back more than his other two remaining defenders.