The Hoops have collected just one point from two games following a defeat against Atletico Madrid in Spain and a home draw against Udinese and head into Thursday's game bottom of Group I - behind their hosts on goal difference.
The Celtic boss knows his side face an uphill task in the competition but remains bullish.
He said: "Realistically, we have to think that to qualify we have to take something from the game and win our remaining two home games and hopefully other teams will take points off each other as well.
"But I know Rennes are an excellent side. I watched them against Lorient at the weekend and they are very exciting going forward but we have exciting players ourselves and we feel we can cause them plenty of problems ourselves."
Rennes coach Frederic Antonetti is set to rest his highly-rated midfielder Yann M'Vila and some of his other regulars.
He said: "We are playing once every three days at the moment and Yann in particular is the player who has played the most in Europe and also in the French national team.
"Not only that, the players need a rest from time to time to keep them fresh for the rest of the season."
Antonetti claims he is unconcerned with Celtic's dreadful away record in Europe.
The Hoops have won only once in their last 31 games on the road in Europe but the Rennes boss said: "I haven't taken a great deal of interest in their previous European experiences.
"I am only interested in what they have done this season. Celtic are a big name in European football. They play football, and not just kick-and-rush football you expect from British sides.
"We have watched them on numerous occasions, as we would with any opponent, regardless of whether it is in the League, the League Cup, or in Europe.
"I personally have only seen them on videos and there is a great deal of physical effort but no lack of finesse, particularly in middle of the park."