Gustavo Raggio's Newell's opened the 2014 season with four wins and four draws but they crashed to a 3-0 defeat at home to Banfield on Monday.
Newell's fell behind after 15 minutes at the Estadio Marcelo Alberto Bielsa, Banfield forward Ricardo Noir finding the back of the net.
Banfield doubled their lead 10 minutes before half-time thanks to midfielder Walter Erviti.
Newell's had a golden opportunity to reduce the deficit six minutes into the second half, but Argentina international Maxi Rodriguez failed to convert from the penalty spot.
The home side were made to rue that chance seven minutes later as Ecuadorian midfielder Juan Cazares sealed the points.
Newell's loss saw them drop to fifth in the table with 16 points from nine games, five adrift of league-leading River.
Banfield improved to 13th spot with 10 points.
River played out a second consecutive stalemate, drawing 1-1 at third-placed Lanus on Sunday.
Lautaro Acosta opened the scoring in the 37th minute, springing the offside trap before placing the ball beneath River goalkeeper Marcelo Barovero after Silvio Romero split two defenders.
River ensured the points would be shared 19 minutes from time, when the league's leading goalscorer Teofilo Gutierrez sensationally beat Agustin Marchesin from long range.
It took the Colombia international's tally to nine goals in as many games.
Independiente rose to second in the standings after accounting for Rosario Central 2-0 on Saturday.
Federico Mancuello and Jesus Mendez were both on target for hosts Independiente in the second half at the Estadio Libertadores de America.
Independiente took a 51st-minute lead through Mancuello, who curled the ball into the top-right corner from outside the box.
Jorge Figal was sent off for the hosts 12 minutes later after receiving a second yellow card but it mattered little as Mendez secured the win four minutes from time.
Independiente's fifth win in six matches saw the club close within three points of River.
In other results, Racing Club made it back-to-back wins.
Racing - fourth in the table - trounced nine-man Belgrano 4-1.
Gustavo Bou and Marcos Acuna scored two goals apiece, while Pier Barrios and Cesar Pereyra saw red.
Boca Juniors returned to winning ways, accounting for Quilmes 1-0.
Copa Libertadores champions San Lorenzo lost for the first time in three matches, beaten 2-0 at home to Gimnasia La Plata.
Estudiantes overturned a first-half deficit to prevail 3-2 over Velez Sarsfield.
Tigre downed Defensa y Justicia 2-1 late on, Arsenal eased past Godoy Cruz 3-0, while Atletico Rafaela played out a goalless draw with Olimpo.
