Arsenal have suffered a 3-2 defeat to Borussia Dortmund in the Emirates Cup. However, what cost them has suddenly become Andrea Berta’s new transfer priority now. MORE ARSENAL STORIESAgainst Dortmund, both Cristian Mosquera and Gabriel Magalhaes made mistakes that led to goals, and that cannot happen when the season starts. Credit: Getty Images/Alex PantlingGabriel Magalhaes and Cristhian Mosquera cannot play togetherIndividually, Gabriel Magalhaes and Cristhian Mosquera are both excellent defenders. Arsenal need a calm and composed defender next to Gabriel – someone who can take control while his partner plays aggressively.