Uruguayan striker Rodrigo Mora stepped up in the absence of star man Teofilo Gutierrez, netting a brace as River accounted for Tigre 2-0 at the Estadio Monumental Antonio Vespucio Liberti on Sunday.
River have collected 16 points from six matches to move three clear of Velez Sarsfield, who played out a goalless draw with third-placed Newell's Old Boys on Saturday.
Gutierrez has played an integral role for River this season, topping the scoring charts with six goals in five games and his red-hot form saw him called into the Colombia squad.
With the 29-year-old away on international duty, River needed a player to take centre stage and that man was Mora.
Mora, who had only netted one goal prior to Sunday's fixture, opened the scoring in the 38th minute from the edge of the area.
The 26-year-old striker volleyed the ball into the corner of the net, leaving Tigre goalkeeper Javier Garcia flatfooted.
Mora doubled the hosts' advantage three minutes after half-time, when he coolly controlled the ball between two defenders and fired beyond Garcia once again.
Tigre played out the final three minutes with 10 men after Martin Galmarini saw red for a second bookable offence.
Boca's resurgence under Arruabarrena continued on Sunday following their narrow 1-0 win at nine-man Olimpo.
The Argentine powerhouses had lost three of their opening four games prior to Arruabarrena's arrival, but Boca have now won back-to-back matches with the former defender at the helm.
After Olimpo striker Miguel Borja was sent off in the 14th minute, Emmanuel Gigliotti made the breakthrough with 12 minutes remaining.
Gigliotti got on the end of a Fernando Gago cross to send a diving header past Olimpo keeper Nereo Champagne.
Juan Sills was sent off in the final minute of normal time for the home side.
In other results, Independiente joined Newell's on 12 points after edging 10-man Banfield 1-0 on Saturday.
Banfield were forced to play with a man less from the 34th minute following Lihue Prichoda's dismissal and it took its toll as Federico Mancuello scored the winning goal on the hour-mark.
Fifth-placed Rosario Central played out a 1-1 draw at home to Copa Libertadores champions San Lorenzo, who dropped to the foot of the table after Quilmes routed Arsenal 4-0.
Lanus climbed up to sixth after coming from behind to beat Racing Club 3-1.
Silvio Romero scored twice for Lanus, while Racing's goalscorer Ricardo Centurion was sent off in the 87th minute.
Elsewhere, Atletico Rafaela were 2-0 winners over 10-man Gimnasia La Plata.
Estudiantes downed Belgrano 3-1, while Defensa y Justicia earned a 1-1 draw at Godoy Cruz.
