Roma have fallen 11 points behind leaders Juventus in the race for the Italian title following a run of four straight league draws in which they have scored just twice.
A goalless stalemate with Chievo on Sunday left Garcia frustrated at what he described as a "terrible performance" and they will need to be much improved in order to see off a Fiorentina team still in the hunt for a UEFA Champions League place.
Garcia's men battled through a fiery last 32 tie with Feyenoord to reach this stage and the Frenchman is hopeful that Thursday's first leg in Florence can provide some welcome relief from their Serie A struggles.
"We're second, and we want to stay there, but to do so we need to do better than [a draw with Chievo]," Garcia told Sky Sport Italia.
"I expect more, both from individuals and the squad, because I know what each player is capable of.
"The club and staff are united. It's important to make clear to the team that we must do better, and we have an opportunity to do that now.
"[On Sunday] I was disappointed, but I woke up [on Monday] motivated and charged. We must work to play a great game on Thursday."
Fiorentina, meanwhile, suffered a huge blow to their Champions League ambitions with a 4-0 humbling at the hands of Lazio at the weekend, leaving them four points adrift of Roma's capital city neighbours in the fight for third place.
But captain Manuel Pasqual has urged Fiorentina - who knocked out Tottenham in the previous round and beat Juve in the first leg of the Coppa Italia semi-final last week - to put their horror show at Lazio behind them.
"Just as we did well not to dwell on some of the big wins we've picked up recently, we need to make sure we put this loss behind us quickly," Pasqual told Fiorentina's official website.
"We need to work hard and look at what went wrong, of course, but the most important thing is to move on.
"We have an important game against Roma on Thursday and then Milan on Monday night. Both are vital if we're to progress in the Europa League and continue performing well in Serie A."
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