Played a day later than initially scheduled due to torrential rain, the Superclasico lived up to its billing but Boca could not shake off its determined rival in a 2-2 stalemate at La Bombonera.
Despite leading twice and having further chances to clinch a first-leg victory, Schelotto's men were frustrated, with their coach now concerned it could cost them.
"We should have won," Schelotto said. "We wasted so many chances, too many.
"I think we were the better side on the field, but River played their game, caused us some trouble on occasion and now it will be to the Monumental to settle it.
"We had the opportunity to go 3-2 up at the end, but [Franco] Armani has made a great save."
It was Dario Benedetto who was denied by Armani late on, but the Boca substitute, who scored his side's second, had a different outlook on the game.
"We got some things wrong, but it is a fair result," Benedetto said. "Maybe it is not the result we were looking for, but we are happy, we played a good game. I do not know if we were better than River."