Nieuwendyk was at the Corel Centre earlier in the day but did not take part in Toronto's optional skate. The Maple Leafs would not disclose the nature of the injury, saying only that the veteran was not 100 percent.
The veteran has been getting treatment for a sore back, according to teammate Brian Leetch.
Sources told The Sports Network of Canada that Nieuwendyk was having an MRI exam on his back.
He scored his second goal of the series in Monday night's 2-0 victory over Ottawa that gave Toronto a 2-1 lead in the best-of-seven matchup.
Information from The Associated Press was used in this report.