Nainggolan joined Inter in June and was reunited with Spalletti, who previously coached him at Roma.
Inter are reported to have paid €23.5 million (£21m/$48m) plus Davide Santon and Nicolo Zaniolo for the 30-year-old, but he has featured in just one of their three Serie A outings so far.
He also scored in that appearance against Bologna, but Spalletti wants more from the Belgian away from the offensive side of the game
"Nainggolan is a strong player, as are all of his team-mates, otherwise they would not play here," Spalletti told reporters ahead of Saturday's clash with Parma.
"He's more striking than just being about scoring goals. He's that kind of player who gives those flashes like in Bologna.
"But then there are other statistics, the consistency he has in the game, and on these numbers he can do a lot better.
"At the end of the game, we look at the performance data. He can give us something more, can improve, but then he is one that can make the difference."
Inter will be out to claim a second successive win when Parma visit, but they will once again be without talented striker Lautaro Martinez, who Spalletti suspects will also miss the Champions League opener at home to Tottenham on Tuesday.
The Argentina forward joined for a reported €23 million from Racing Club in pre-season, but injuries have disrupted his start to life in Serie A, with a haematoma in his calf – which prevented him playing for his country during the international break – causing him discomfort.
"His situation is improving," Spalletti said. "He is healing, but we still have to evaluate him.
"For Tottenham we will have to see. Personally, I think it's unlikely because they are problems that, if not healed completely, can lead to a long lay-off, but we are still calm."