Reds boss Rafa Benitez is desperate to find a buyer for Riera.
"I can tell you why the transfer didn't happen," former Russia midfielder Karpin told Russian website Championat. "I understand that the player wanted to have some match practice before the World Cup.
"In this situation, I would have even agreed to play free of charge.
"But Riera wanted to go to Spartak in order to hit the jackpot. We made him understood that this wouldn't happen with us. I can say that there will be no acquisitions before the transfer window closes."