Leonardo Ponzio - This #5 just arrived at River in January, but has a pretty impressive history: He won the U-20 World Cup with Argentina in 2001, and was transferred from Newell's to Zaragoza of Spain, where he was a starter for 3 years. This may seem like a some-what long career, but Leo barely turned 25 a few weeks ago. He has a great shot accuracy and recuperates the ball a lot. Not only that, but he is multifunctional, which makes him another option to play as a right defender or midfielder.
Oscar Ahumada - A good variation for the position. Despite Ponzio being the starter, this hasn't stopped him from playing: he came in as a substitue druing the second half in most games. A great option to switch off with Ponzio during this tough schedule.
Nicolas Domingo - The future #5 of River. This guy has serious potential, and I've been saying for a long time that he is the next Mascherano. He proved my words when he played in most games in the 2006 Apertura and was able to shine in important games, like the one vs. Boca, when outplayed and outclassed Gago. :)
Rene Lima - A youngster made in River's lower divisions. He was able to play a few games last season, but probably won't receive too many minutes this season, if any at all, due to the many options in his position.