River Plate could face a Monumental Stadium ban after a fan threw a piece of wood and hit Godoy Cruz defender Leandro Grimi as he celebrated scoring the equaliser in his team's 2-1 upset win on Wednesday.

Godoy Cruz claimed three valuable points in their fight against relegation with a winning goal in added time from Julio Rodriguez when River were down to 10-men after the 68th-minute dismissal for two bookings of Colombian defender Eder Balanta.

River issued a statement on Thursday condemning the incident and said they were working with state and private security bodies to find the culprit and ensure he was banned from the club.

"You can't punish 50,000 fans because of one idiot who causes damage. He evidently broke off a part of the stand because he didn't get in (to the stadium) with that (wood)," said club president Rodolfo D'Onofrio.

"We're searching for him on the cameras to identify him and apply the maximum sanction... to expel him. We don't want people like that coming into a football ground ever again," he told TyC Sports.

Grimi, who had spells with AC Milan, Portuguese club Sporting and Genk between 2007 and 2013, was unhurt and made light of the incident.

"Unfortunately here in Argentina because of (the actions of) a few everyone has to pay. They were probably 10 (hooligans) and then all River's fans can't come to the ground," the defender told reporters anticipating a stadium ban.

The defeat was a heavy blow for River, who had taken an early lead through striker Fernando Cavenaghi after a promising start to the Final championship, the second of two in the season.

Balanta's red card and injury to right back Gabriel Mercado, one of River's best players this year, unbalanced the defence.

Fresh optimism blew through the club when D'Onofrio won the presidency in December, ending the controversial four-year reign of former captain and coach Daniel Passarella, who oversaw River's humiliating first relegation in 2011.

Ramon Diaz was retained as coach despite a poor Inicial championship showing in which River finished second-bottom.

River began the year with two friendly wins over arch-rivals Boca Juniors during the summer break and a 1-0 home victory against Gimnasia at the start of the Final.

They followed that with a 1-1 draw at Rosario Central last weekend but are now five points behind leaders Estudiantes, who have a maximum nine points from three matches after Wednesday's 2-1 win over Lanus in La Plata.

Title holders San Lorenzo are three points back after a second consecutive win, 2-0 away to Argentinos Juniors at the Diego Armando Maradona ground.

Survival-specialist coach Ricardo Caruso Lombardi got off to a losing start on his return to Quilmes, who have charged him with saving the team from relegation.

Quilmes lost 2-0 at home to Colon, who are also in danger of the drop, and tumbled into the relegation places.

Relegation in Argentina is decided by a separate table of teams' average points over three seasons with the bottom three, currently Argentinos, Quilmes and Godoy Cruz, going down in May. Colon are one place outside the danger zone.

Image: River Plate fans

Photograph: Julian Finney/Getty Images