Despite being a key figure for the Serie A club in recent seasons - helping Napoli to the Coppa Italia last term - Hamsik's form has dropped off somewhat in this campaign.
The Slovakia international was reported to be close to leaving the Stadio San Paolo during the close-season and remains the subject of rumours after a difficult start to 2014-15 for the midfielder.
Hamsik faces former club Slovan Bratislava on Thursday with Napoli already qualified for the UEFA Europa League knockout stages, although a draw could be enough to win the group.
Benitez confirmed the 27-year-old will start the game in Bratislava as he bids to rediscover his form.
"I have full confidence in Marek and I am certain that he will return to his full potential," Benitez said.
"As for me, my choice is always to trust people on the pitch and I can tell you that tomorrow the team will be Hamsik and 10 others.
"Against Slovan we will give our best, as long as we play shoulder to shoulder, we can go a long way."
Thanks to Napoli's strong head-to-head record against Sparta Prague and Young Boys, the Italians will progress top with a victory or a draw if the other game is a stalemate.
The former Liverpool boss feels Napoli will have a greater chance of threatening the latter stages of the tournament by finishing top of Group I.
"The goal now is to win the first place in the group, then we will look forward. We want to give our best in Europe," he explained.
"I have said many times that the next game is always the one that matters most. We are focused to look for success and first place that could be very useful in the draw.
"Meanwhile, we will give our all to continue in Europe because it is a journey that we want to continue to the end."
