The Belgium international penned a season-long loan deal from Cagliari on Tuesday, with a view to a four-year permanent switch.
And, after making his bow in the Coppa Italia in midweek, Nainggolan made a perfect start to life in the Italian capital, turning in a fine performance as Roma provided a resounding response to last weekend's thrashing at the hands of Serie A title rivals Juventus.
A delighted Garcia said: "Radja had a fine game, but everyone did this afternoon.
"It's easier to settle into a team like this when Kevin Strootman and Miralem Pjanic are playing at this level.
"Nainggolan knows how to do everything.
"He has technique, plays the long ball, can push forward and shoot for goal. Today his role was more about providing balance to the team.
"He's incredible, it's as if he has been with us since the start of the season."
Garcia hinted that he would not look to bolster his squad further in January, instead preferring to wait for injured and suspended members of his squad to return to the fold.
"I am happy with my squad," he said.
"We had to wait for a few things, now we know Mattia Destro is back and Francesco Totti has rediscovered his fitness levels, so we must wait to see how Federico Balzaretti is doing.
"Without injuries, we are a strong side and today had Daniele De Rossi, Leandro Castan and Adem Ljajic suspended."
Roma stayed eight points behind the leaders after Juventus triumphed 4-1 over Cagliari.
