Clubs that aren't affected by injuries to their main strikers are those that have backup players with similar quality to their main players, so they don't rely too heavily on their first-team players. Alonso himself must also rely on other players to sharpen Real Madrid's attack. During the Club World Cup, Real Madrid didn't have a particularly impressive performance, meaning they couldn't achieve much in that competition. Therefore, Alonso should see this as a valuable lesson for further improvement before next season resumes.
I think it is not possible for Real Madrid. Because finding alternatives to their main strikers may not be an easy task. Finding alternatives to Kylian Mbappe is not an easy task for Alonso at the moment. It may not be easy for Alonso to keep alternatives to Vinicius Junior and Rodrygo. Real Madrid already have Endrick and Gonzalo Garcia as alternatives to the main strikers, who may be able to meet Alonso's expectations. But I do not consider them as worthy alternatives to the main strikers yet. Xabi Alonso must qualify those alternatives or sign worthy players.
Speaking of alternative strikers, perhaps Xabi Alonso worth a try his luck by borrowing an experienced player from another club instead of signing him permanently, as Ancelotti did with Joselu. I don't think that's a bad idea. However, if Real Madrid are looking for a backup for Mbappe, they'll have to sacrifice Endrick or Gonzalo Garcia. Overall, I don't think Real Madrid have a significant problem up front, Xabi Alonso has a number of players who can switch roles and play as center forwards.
The main issue that Xabi Alonso should seriously address is the defense, which so far hasn't looked very solid. Having a sharp attack is pointless if the defense is porous, it won't lead to success. Therefore, Xabi Alonso must find a solution to encourage Raul Asencio, Dean Huijsen, and Trent Alexander-Arnold to reach their best potential.