Real Madrid got Alaba from Bayern Munich in the summer when they let Sergio Ramos left Bernabeu and joined PSG. He is a replacement directly for Segio Ramos as one of two central defenders in their squad. This injury will put Real Madrid into a situation they must to find for a temporary replacement for Alaba in the winter transfer.
Alaba's injury is considered quite serious, with a torn anterior cruciate ligament in his left knee, and he will undergo surgery in the coming days. This is an extremely heavy loss for Real Madrid as a torn ACL is the worst injury for an athlete. Alaba is expected to be out for 7-9 months, and the season is practically over for him from this December.
The absence of Alaba, I believe, will seriously affect Real Madrid's plans for the remainder of the season. He is currently the team's primary left-center back, pairing with Antonio Rudiger after Militao's long-term absence. Now, with Alaba's injury, Real Madrid has only two central defenders in the first team, Nacho and Rudiger. This is a thin squad, especially as they approach important matches in early 2024, such as facing Atletico Madrid twice in the Spanish Super Cup and playing against the league leaders Girona (February 11), not to mention the Round of 16 in the Champions League.
It's likely that midfielder Aurelien Tchouameni will play as a center back in place of Alaba. Therefore, Real Madrid is likely to consider finding a replacement in the January transfer window to strengthen the defensive line.