The Belgian has arrived on an initial six-month loan.
"What does Nainggolan offer us? We can talk about him because he's a Roma player now and I can say that we'll be a stronger team with Radja," said Garcia.
"He's not just a signing for the second half of this season but for the future too - we hope to be playing in Europe next year so having another quality option is a good thing.
"Pjanic? Nothing changes for Miralem. Where could Nainggolan play? He can play in all three midfield slots. He's an all-round player.
"Play four midfielders together? That's possible because Pjanic can play higher up the field and Radja also has attacking abilities.
"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."
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