The record 33-time Argentine league champions went down for the first time in their 110-year existence on Sunday when they lost a relegation-promotion play-off against Nacional B side Belgrano 3-1 on aggregate.
"For this transitional new stage Matias Almeyda was unanimously appointed to take charge of the new River Plate team," the board said in a statement on the club's website.
Almeyda, who played for Parma and Lazio in Italy and Argentina at the 1998 and 2002 World Cups, was a part of Lopez's team this season but missed Sunday's decisive second leg, a 1-1 draw at the Monumental, through suspension.
"The board of directors met today... to unanimously seal the continuity of president Daniel Passarella for the start of River Plate's refounding," the statement said.
Passarella has been heavily criticised by opponents in the huge membership of one of Argentina's biggest and most popular clubs for the way he has handled their massive debts during the team's plunge into the relegation zone.
Hooligan fans known as 'Barra bravas' caused mayhem inside and outside the giant Monumental stadium after Sunday's match, wrecking everything they could lay their hands on and leaving a trail of injured supporters and police officers.
Doubling as Argentina's national stadium, the Monumental is due to stage the July 24 final of the Copa America, which kicks off on Friday with the opening match between the host nation and Bolivia in La Plata, capital of Buenos Aires province.
