Roma progressed with a 2-1 win in a fiery contest at De Kuip that saw two Feyenoord players sent off, completing a 3-2 aggregate victory.
However, Garcia's men have not won in Serie A in over a month and were beaten 2-0 by Fiorentina in the Coppa Italia last month, having claimed a 2-0 win and a 1-1 draw in the two league fixtures with the Florence club.
"We've met Fiorentina three times already and I think tomorrow will be different to the other games," Garcia said.
"We need to think of this as a match lasting 180 minutes: that's crucial if we're to play well and go through. We have to play better than we did against Chievo [in Sunday's 0-0 draw], but that shouldn't be too hard.
"I hope to see the Roma of Rotterdam, of the first half against Hellas [Verona] and the last 20 minutes against Juve [in a 1-1 draw nine days ago].
"I'm confident in my players' ability: they're quite capable of playing as well as they did until not long ago.
"My players know we have to take the game by the scruff of the neck, playing to our strengths."
Speaking of their recent poor form, forward Adem Ljajic added: "There's no mental block. We're a great group and we talk together. We'll come through this with our heads high.
"We'll all behind the coach and trying to reproduce the things we run through in training during the week.
"We played our game for the first six months. We're doing less well now and we need to get back to our best."
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