The Superclasico tie is locked at 0-0 ahead of Thursday's second leg at the Estadio Monumental Antonio Vespucio Liberti.
But River's form is worrying for Gallardo and their chances of reaching the final.
They are winless in five matches in all competitions, including going down 1-0 to Racing Club in the league on Sunday.
That loss saw River slip from top spot in the Argentine Primera Division, with Racing moving top by two points.
Gallardo is eyeing improvement when his side face rivals Boca.
"We have to make a great game against a lifetime rival," he told the club's website.
"We imagine greater control of the game."
With their chances of winning the league seemingly slipping away, River need a result against Boca.
In contrast, Boca are in good form heading into the clash - coming into the encounter on a five-match unbeaten run in all competitions.
Rodolfo Arruabarrena's men will be hoping Jonathan Calleri can rise to the occasion after his brace against Independiente in the league on Sunday.
The other semi-final will see Atletico Nacional travel to Sao Paulo with a 1-0 lead.
A Luis Ruiz goal led the Colombians to their first-leg win and it will be a handy advantage ahead of the meeting with the Brazilian Serie A side on Wednesday.