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These are some really important issues about Xabi Alonso tactics and recent loss to PSG. Team defense clearly has problems letting in four goals. You are right that having too many attacking players can make defense weak. It also looks his three defender plan is not working, showing he is still trying to find best way to use his players to make them strong everywhere especially in defense. It will be interesting to see how he fixes these defensive issues.
In my opinion, Real Madrid's defense is very weak at the moment and if a team's defense is so weak, then the overall performance of that team is sure to be bad. When the defense is in good condition, the opponent's attacks can be easily repelled. Real Madrid's defense performed so badly in the last match that the strikers and midfielders did not get any chance to bring the team back into the match. I think the Real Madrid manager should definitely think carefully about their defense. If their defense line-up does not change much in the new season, then they will again be a failed team in all kinds of tournaments only because of their defense. Before Real Madrid's defeat by PSG in the Club World Cup semi-finals, Real Madrid had a bad experience against Barcelona in La Liga and that bad experience was in all the El Clasico matches last season, but Real Madrid was defeated. They must work on these weaknesses and return to good performance.