Last season's Ligue 2 champions have yet to taste defeat in Ligue 1 this season, winning five and drawing two of their seven matches to sit atop the table, but Ranieri is wary of the defensive strength posed by a Reims side which has only lost once so far this campaign.
"Reims have the second best defence in Ligue 1," he told a press conference on Friday. "Their organisation is very good.
"They beat Lille and Olympique Lyonnais and have lost just once, it will be difficult.
"Moreover, this game comes after three games in a week. Concentration will be very important.
"We conceded too many shots against Bastia, we must work on that too."
Monaco followed up their 1-1 draw at champions Paris Saint-Germain with a 3-0 win over Bastia on Wednesday to maintain a two-point lead at the top of Ligue 1. Reims sit in 10th place with 10 points from their opening seven games.