It was a fine result for Lanus coach Guillermo Barros Schelotto, a Boca idol in his days as a winger who masterminded the victory against his old club.
"I didn't enjoy it fully like I would beating another team," the former Columbus Crew forward told reporters. "We were always in control, we deserved to win."
Ustari spilt a high ball into his penalty area, defender Rolando Schiavi's attempted clearance hit the bar and striker Mario Regueiro put Lanus ahead after half an hour.
Despite Uruguayan Regueiro's red card in the 81st minute for a second booking, Lanus increased their lead in the 86th through Diego Valeri in a counter-attack.
Former Paraguay World Cup coach Gerardo Martino's unbeaten Newell's team, who won 2-1 at Colon, lead the Inicial championship standings with 18 points from eight matches.
Fans are demanding a change of coach at River where they see the inexperienced Matias Almeyda struggling to build a solid team and chanted at the end of Sunday's defeat for the return of the highly successful Ramon Diaz.
San Lorenzo's coach Ricardo Caruso Lombardi, who normally finds something to complain to the referee about, tried to put a lid on his frustrations after a second successive defeat, saying: "I look like Snow White I'm so calm, but inside I'm a volcano.
"At big clubs, time is shorter [but] while I feel good with the players, I'll carry on. They support me."
