The combative midfielder opened the scoring for Rafa Benitez's men, who face Swansea in the Europa League on Thursday, before the winger put the match to bed in the second half
Blerim Dzemaili gave the visitors the lead with a well-taken volley in the opening 45 minutes before Lorenzo Insigne made it two after the break with another superb finish.
Napoli started the match the better side of the two and got their first chance of the game after nine minutes of play. Marek Hamsik went past his marker before trying his luck, but his dangerous shot went just wide.
Gonzalo Higuain then came close to breaking the deadlock around the half-hour mark, only to be denied by a fine reflex from Gianluca Pegolo.
The Sassuolo goalkeeper was again called into action shortly after when Lorenzo Insigne threatened, yet the Italy international's left-footed striker was kept out by Pegolo, too.
Nevertheless, there was little Pegolo could do about about Dzemaili's powerful shot from outside the area in the 37th minute, as the Swiss midfielder netted his fifth goal of the season with a fine volley after good work from Hamsik.
Napoli continued where they left off after the interval and Insigne doubled his side's lead in the 55th minute with a superb finish. The tricky attacker cut inside from the left after being set up by Hamsik and beat Pegolo with a curling effort in the far corner.
The home side nearly pegged one back with 20 more minutes to go via Simone Zaza, but the 22-year-old aimed just wide.
Insigne almost doubled his personal tally in the closing stages of the game after being set up by Goran Pandev, but the winger's attempt went inches past the near post.
Napoli sit third in the Serie A table following this weekend's results with 50 points from 24 games, holding a six-point lead over fourth-placed Fiorentina.
Rafa Benitez's men take on Swansea in the Europa League in midweek before resuming Serie A action at home against Genoa on February 24.