Nainggolan completed a move to the Stadio Olimpico on Tuesday, initially joining from Cagliari on loan with Roma having an option to co-own him at the end of the current campaign.
The Belgium international has made 17 appearances this season and could go straight into Roma's midfield for their match with Genoa on Sunday in place of the suspended Daniele De Rossi.
Head coach Garcia was delighted to have acquired Nainggolan, and feels the 25-year-old will give his side a new dimension in midfield.
"I can say that we'll be a stronger team with Radja," the Frenchman said.
"He's not just a signing for the second half of the season but for the future too - we hope to be playing in Europe next year so having another quality option is a good thing.
"He can play in all three midfield slots, he's an all-round player.
"I'd rather not change our formation but it's good to have more options. We'll be stronger with Radja for next season too."
