Well, first
Aurelio is from Valencia. So Rafa knows very well of him and his ability.
I think the reason lies in his versatility, and who can play better than him in the current squad. As a left back, he is a 2nd choice of
Riise, ahead of Warnock. As a left wing, his main competition is M.Gonzalez.
L.Garcia is not really a wing player, and
Gerrard plays worst in the left wing position across all positions in midfield.
Not sure when can
Kewell can join the squad this season. But even if he's here, Aurelio offers better crossing quality than Kewell. So far, Aurelio showed higher confidence in 1-on-1 situations against defenders. And he tends to go deep down the flank before making the cross.
One drawback is Rafa needs to be cautious if he uses both
Agger and
Aurelio on the back together. Both love getting the ball on foot and push forward. It's been showing Liverpool's left side is often the point to attack whenever Aurelio played left back. Still, personally I love these 2 players as their confidence when having the ball is rare among Liverpool players these years.
One last point.
Aurelio had a history of injury problem from Valencia time. I'm not sure how bad was it, but again Rafa needs to keep an eye on his fitness level as the season proceeds.
