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 37-year-old has no coaching experience having come out of semi-retirement playing for Lyn in Norway and then Fenix in the fourth tier of the Argentine game to return to River, his first club, in 2009.
The board also ratified Daniel Passarella, who took office 18 months ago, as the club president charged with leading River into unknown territory in the tough second tier of Argentine football.
"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.
River did not have a bad season but paid the price for a poor points average over three seasons - the system by which relegation is determined in Argentina.
The system was, ironically, created in the early 1980s to protect the big clubs from a poor campaign.
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.
The Monumental has been closed on orders from a prosecutor seeking to establish if too many match tickets were sold. He will also investigate the policing operation which involved 2,200 officers. Police made nearly 40 arrests.
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.
