The striker scored twice in Thursday's 3-0 win over Levante to book a semi-final clash against Barcelona, but the Argentinian remains focused on league success.
Valencia are currently third, 14 points behind leaders Real Madrid, while Racing are 16th despite a run of good results under new coach Juanjo Gonzalez.
"The league is very important to us and we must remain consistent," Piatti told www.as.com. "We need a win [against Racing].
"I always said that mentality is a very important factor. I never stopped working and trying, sooner or later results appeared.
"I have a lot to prove in Valencia. It was a good game all round against Levante."
Racing were five games unbeaten under Gonzalez, who replaced Hector Cuper, but that came to an end in the 2-1 home defeat by Getafe last weekend.
The Green and Whites were beaten 4-3 by Valencia in a dramatic clash on the first day of the season when Roberto Soldado struck twice in the final three minutes to complete his hat-trick and snatch a late win for Los Che.
However, after a poor first half of the season, Gonzalez has steered Racing out of the relegation zone with the club securing 11 points from a possible 15, and knows the threat facing his side at El Sardinero.
"Valencia is a great team with great individuals in all parts of the field," Gonzalez told www.as.com. "We have to stop and attack as a group."