The Serie A outfit host Manchester City on Wednesday knowing a win would see them qualify from Group F alongside Bayern Munich, while a goalless draw would be enough should CSKA Moscow fail to beat the pool winners.
Garcia paid tribute to his side's efforts so far, and says the clash will City will be a "cup final" for his side.
"We should have been out of the running by this stage because we were in the fourth pot and yet we're still in the hunt," he said.
"Credit to us that we still have the chance to play this cup final in front of our fans at a passionate Olimpico.
"We need to make the most of every moment. We're looking to pull it off, a win would be incredible.
"We know that Manchester City are a good side. It will be very tough tomorrow but I'm sure we can do it.
"We can only decide our own fate with a win. We can't know what happens in the game between Bayern and CSKA.
"So our only option tomorrow is to score one goal more than City to win the match."
Roma earned a 1-1 draw in Manchester in September, and will have been boosted by the news that City striker Sergio Aguero will miss the clash with a knee injury.
"We did well in the first game, but it'll be different tomorrow," he added. "City are back on track in the league and we're at home.
"Aguero's absence is sad news. Great players like him should be involved out there."
