All the goals came in the final 25 minutes as substitute Cristian Nunez and striker Joaquin Boghossian put Newell's two up before Ruben Ramirez pulled one back.
Banfield, who could have gone top with a victory, are two points behind co-leaders Colon and Godoy Cruz, the only remaining unbeaten teams after six matches.
Newell's, four points off the pace, have a game in hand.
All the weekend's matches in Argentina began after a minute's silence for the hundreds of people who died as a result of the earthquake in neighbouring Chile.
River's arch-rivals Boca Juniors, who have also been going through a lean spell for the last year, drew 1-1 at home to Estudiantes on Friday when striker Martin Palermo's penalty was cancelled out by an injury-time volley from Leandro Gonzalez.
Boca and River are in the bottom half of the standings as are Racing Club, beaten 1-0 at arch-rivals Independiente in their derby, the "Clasico de Avellaneda," on Saturday.
Velez Sarsfield lost the chance to go top when they were beaten 3-2 at Huracan.
