It's more than just Mexes-Juan, but seriously... the only teams who have given up more goals than Roma right now are Torino, Reggina, and Cagliari -- bastions of Serie A defensive units.
Roma are a mess. Mentally, psychologically and morally. That happens when things don't go their way. When they start the season badly, when their most important player, Totti is missing, now their second most important, De Rossi. Plus Mexes only played two of those games. They won one and nI can't remeber what happened in the other. Juan played none.
Not to mention that a midfield of De Rossi, Pizarro/Aquilani, Perrotta, Taddei and Menez, wheer De Rossi is the only really capable defensive player there doesn't help. Riise sucks, Cicinho and Cassetti are terrible defenders. I feel sorry for Roma actually.