"When you get to the semi-final you have to make the most of your luck," Valverde told a news conference on Wednesday. "A semi-final, it's the chance to play a final and we must give everything to get there.
"Valencia will do the same thing. Reaching a final doesn't happen every day," added Valverde, whose side have already beaten Celta Vigo and local rivals Espanyol in the cup this month.
Barcelona, the record 29-time champions, are attempting to become the first team to lift the cup four years in a row since Athletic Bilbao in the 1930s.\
"They're feeling the effects of lots of matches this January," said Valverde, who spent six months in charge of Valencia between 2012 and 2013.
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Barcelona thrashed Valencia 8-1 on aggregate -- following a 7-0 win in the first leg -- at the same stage of the competition two years ago, but Valverde acknowledged a repeat scoreline is unlikely.
"It won't be like the last time, a 7-0 is not normal for a first game. Valencia were in a delicate position then, but now they have a team that is good enough to win any match."
Colombian international Luis Muriel gave Sevilla the lead after 21 minutes, scoring with the aid of a deflection off the post.
But Leganes levelled on the night when Sevilla goalkeeper Sergio Rico blundered trying to keep out a header from Greek defender Dimitrios Siovas.
Leganes, who were playing in the third division five years ago and are in the Copa del Rey semi-finals for the first time in their 90-year history, remain the outsiders against Sevilla, at least on financial terms.