Italy international Destro departed for Serie A rivals Milan on Friday, the striker initially joining on loan with an option to make the move permanent.
Garcia insisted earlier this month that Destro would not be sold and the striker's departure has left a void.
The Frenchman insists a replacement will be brought in, however, and wants his team to focus on Saturday's clash with Empoli at Stadio Olimpico.
"We'll sign a striker to replace Mattia Destro," Garcia said.
"Clubs should only sign useful players that are going to add something to the group.
"There's a lot of talk about transfers at the moment, but the players know they need to focus on Empoli."
Roma, who are second, have slipped seven points adrift of leaders Juventus after a run of four draws in five league matches.
Garcia has warned of the dangers posed by an Empoli side that Roma have beaten in Serie A and the Coppa Italia this season.
"We're in the hunt for three titles and the numbers don't lie for my team," he added. "Let's not forget what we have achieved.
"Empoli are the most well-organised side in the league, we did a great job to unlock them in our last two matches.
"We need to be alert on dead-ball situations and attack at just the right moment to score."
