Boca's home clash against Racing was suspended on September 14 due to torrential rain with Rodolfo Arruabarrena's men leading 1-0 after 56 minutes.
The top-flight fixture was rescheduled to Thursday and Boca had no answer as striker Gustavo Bou bagged a quick-fire brace for Racing at La Bombonera.
Boca went into the match with a one-goal lead thanks to Jonathan Calleri's opener almost a fortnight ago.
But Racing flourished in perfect conditions in Buenos Aires thanks to Bou, who fired home a half-volley to equalise five minutes after the restart.
Bou was at it again nine minutes later, glancing a header past Boca goalkeeper Agustin Orion - taking his tally to three goals in less than a week.
The defeat handed Boca their first loss under coach Arruabarrena and left the club languishing in 11th spot with 10 points.
Racing are three points better off after ending their three-game winless streak, though they are still seven adrift of league leaders River Plate.
River were held to a 1-1 draw by Arsenal in Thursday's rescheduled fixture.
The Argentine giants and Arsenal had their September 14 meeting postponed without a ball being kicked because of heavy rain.
Over a week later and the two teams were forced to share the spoils.
Hugo Nervo gave Arsenal a 50th-minute lead via the penalty spot, but Leonardo Pisculichi hit back within a minute as River secured a draw at the Estadio Julio Humberto Grondona.
