The Celtics were beaten 104-96, but are still three wins clear at the top of the Atlantic Division.
The head coach told reporters: "They played great tonight. Played really hard, really together. They took us out of what we wanted to do.
"Like I said, we weren't playing the New Jersey Nets that played without D-Will for those 10 games. That is their team and they're a good basketball team."
The Celtics face the Los Angeles Clippers tonight.