The Naples-based outfit are in good form heading to San Siro, unbeaten in 11 matches in all competitions.
But their past three league matches have ended in draws, seeing them slip to fifth in the table.
Benitez is wary of the quality in Milan's squad, even if Filippo Inzaghi's men are struggling in seventh.
"The Milan match will be very difficult, as we know they have quality players who can make the difference in a moment," he told Sport Mediaset after his side's 3-0 win over Slovan Bratislava on Thursday.
"We have faith in ourselves and know we must avoid the mistakes we've made in the past."
With seven goals, Gonzalo Higuain has also been good this season - but he is also goalless in December.
"It's too early to say," the Spaniard said.
"The president has shown he is active on the market and we’ll see what happens."
While Napoli have struggled to score away from home, Milan's goalscoring issues have been in front of their own fans.
Injuries have tested Inzaghi's side, particularly in defence, as they look to bounce back and stay in touch with the top four.
