Chelsea boss Antonio Conte has confirmed he wanted to sign Roma midfielder in the summer.
Chelsea meet Roma in the Champions League on Wednesday night.
"It's not right to speak about players from another team," Conte said.
“Yeah in the past we tried to buy him but now Nainggolan is Roma's player and I have great respect for him and for Roma, and also great respect for my players.
“It's not right to speak about another team's players. I think he's a great player, for me he's one of the best midfielders, but he's an opponent."
Conte has been linked with Chelsea's Champions League opponents in the past.
“Obviously Roma are a great team, so I'm very proud my name was linked to them," Conte replied.
“They chose two excellent Coaches, Spalletti first and then Di Francesco."