The Hoops must better the Portuguese side's result against Barca when they face Spartak Moscow on Wednesday night if they are to reach the last 16 of the tournament.
Barcelona have already qualified but Iniesta insists the Catalan giants will not sit back when they host Benfica at Camp Nou.
The playmaker is keen to help team-mate Messi, who has already voiced his hope that Celtic make the knockout rounds, smash Gerd Muller's goals record of 85 for club and country in a calendar year.
Iniesta said: "Barcelona always enter the field to win. We've never gone into a match not trying to win - especially in the Champions League.
"If the Celtic people believe we're treating the Benfica match as a friendly they are very much mistaken.
"Barca have many things to play for. Principally, the image of our club, but also our supporters."
He added of Argentinian superstar Messi, who is one goal short of Muller's record: "We want to help Leo to get even more goals in the year and beat the record."
"Benfica are a strong team and it was difficult for us when we played in Lisbon.
"That is a warning and we know we'll need to work hard in the Nou Camp.
"They are trying to qualify with us but Celtic shouldn't fear we might relax."