Wow, this is potentially a huge blow and definitely gives the Heinze talk some merit. Though frustrating to watch at times, Aurelio was beginning to show signs of competence before going down with a devastating rupture of his achilles tendon back in early April against PSV. It could be early October before he would be able to return.
That leaves us with just John Arne Riise at left back, which is fine, but if something were to happen to him, we only have an out-of-position Arbeloa to cover. It would be great if we could pick up a player who was as versatile as Arbeloa, preferably one not named Heinze, who I would never want starting in the middle.
Next offseason, or even the January transfer window, I expect us to go fishing for defenders. With Hyypia probably leaving after next season, Carra reaching 30, and Finnan not getting any younger, we're setting ourselves up for a crisis. What might make it worse is having a new kid, Babel, play in front of Riise. Knowing that Riise likes to go forward too, it could make for an abandoned flank.
Edit: I guess I should add that we also have Emiliano Insua, if needs be. In the few matches he played last year, he looked capable though I would hope he learned to clear the ball better and to stay back if he wasn't going to get back faster. Most of his problems lie with his youth though, and should be things he can easily improve. He clearly has all the other attributes to play for us, but his loan ends after next season and it's unclear whether or not we will sign him, but I think we should try.