Napoli, who host Cesena on Wednesday, have won just one of their last four Serie A games and taken just two points from their last three matches.
Their last game of January also ended in defeat, a 3-2 loss at Genoa, that left the club seventh in the table, nine points behind third-placed Udinese, who hold the last Champions League qualifying spot.
"In this month of January we've suffered greatly, now we'll try to roll our sleeves up, making an effort from now until May," Mazzarri said. "We've had a very competitive calendar and we are not used to that."
Napoli will be without striker Ezequiel Lavezzi, who serves a one-match ban, and injured captain Paolo Cannavaro, but leading scorer Edinson Cavani will return to the starting XI after being rested on Sunday.
Cavani will partner Marek Hamsik and Goran Pandev up front.
Cesena travel to the San Paolo stadium on the back of a 1-0 loss to Atalanta.
A Marco Rossi own goal condemned Cesena to their fifth loss in six league games and dropped Daniele Arrigoni's side to second from bottom, four points adrift of safety.
"I certainly believe we can defend our Serie A status," Arrigoni said. "But in order to do so, we will have to play much better than we did against Atalanta. We all believe it's possible to avoid the drop.
"It will be easy to prepare for the game at Napoli as everyone thinks we don't have a chance against them. But just when we appear to be dead, we might revive."
Napoli have not lost at home since a 3-1 defeat to Roma on December 18.