A Mario Mandzukic equaliser 18 minutes from time gave Croatia a 1-1 draw against Italy Thursday, the second time in two games the Italians conceded an equaliser after the hour mark, following Cesc Fabregas' 64th-minute equaliser in the opening 1-1 draw with Spain.
"Looking at the two games, there has been a notable drop in our level on 60 minutes," Prandelli said.
He said the remedy was simple.
"For the first 60 minutes, we were very well organised, there was not much space between the front line and the defensive line, but when the game gets stretched and there is more distance, that's when we start to struggle."
"I don't think we lack character, I think we lack fresh legs."
Forward Mario Balotelli was substituted on the 70th consecutive game also due to the lack of fitness, Prandelli said.
With four points from two games, Croatia have a strong chance of reaching the quarterfinals, while Italy must beat Ireland Monday to avoid an embarrassing early exit.
Ahead of Euro 2012, Italy suffered three defeats in a row for the first time since 1986, with 1-0 friendly defeats to Uruguay and the US, and a 3-0 thrashing at the hands of Russia.