“I wasn’t happy with the draw, because in my view Liverpool were the toughest team out of the three,” he told HLN.
“It’s not that they have more quality than Real Madrid or Bayern, but they put a lot of hard work in and have nothing to lose.
“Real and Bayern might’ve underestimated Roma, but Liverpool are warriors, a bit like us.”
Tuesday's clash between the two sides also represents a chance for red-hot Reds forward Mohamed Salah to face his former club, and Nainggolan admits that he has been in contact with his ex-teammate.
“I spoke to Salah and he couldn’t believe we’d beaten Barcelona either! He was joking on Instagram that we’ll never hear the end of Kostas Manolas saying he scored the winning goal,” the 29-year-old said.
“It’ll be nice to see Momo again, we had a good rapport. He’s a good guy, sweet and respectful, as well as being an excellent player.
“I am not surprised by his success at Liverpool, as he always had the quality.
“The only difference now is perhaps he’s got more opportunities and also learned to keep a cool head in front of goal. I’m a fan of his.”
Roma travel to Anfield ahead of a 19:45 (UK) kick-off on Tuesday in the know that their opponents let slip a 2-0 lead over bottom-placed West Bromwich Albion on Saturday, leaving the Hawthorns with only a point.