The 54-year-old highlighted the importance of the upcoming encounter and refused to criticise Mario Balotelli's performance against Spain at the weekend
The Azzurri kicked off their campaign with an impressive 1-1 draw against defending champions Spain, and will now lock horns with Slaven Bilic's men, who defeated Republic of Ireland 3-1 to jump to the top of Group C.
"We have not solved our problems, we only fielded a team which could fight for every ball," Prandelli told reporters at a press conference.
"This will be a match decided by details, but also by the mentality and physical condition of the players.
"We have two days to prepare for this match, which I believe will be the decisive game of the group.
"My plan is not to make many changes but to continue with the same plan, but first I will need to have a full report on the physical condition of every player."
Prandelli then played down the criticism targeted at Mario Balotelli, who cut a frustrated figure against Spain before being replaced by Antonio Di Natale.
"Balotelli is a 21-year-old boy who is on the road to find maturity," the Italian coach explained. "What I ask from him is to play simple, he must not try to win us the match every time he has the ball.
"In some moments, he played simple, but in others he worried too much about finding his position."
Questioned about who will lead the attack against Croatia, Prandelli said: "Right now, I do not know.
"First, we must decide how we will take on Croatia. They are a great side. When we decide this, then I will decide which striker will start."