The Brazilian, who joined the Turin club from Palermo in the close season in a 22.8-million-euro deal, was in lethal form in the first half of the season, notching 11 Serie A goals.
But he has only scored once in 2009, an equaliser in a 1-1 draw with Sampdoria last month, leading to speculation that controversy over whether he should play for Italy or Brazil is distracting him.
Amauri hit back at Gattuso last week saying he did "not care" what his possible future Italy team mate thought.
Brazil coach Dunga called Amauri up for last month's friendly against Italy as a replacement for the injured Luis Fabiano. But Juventus refused to release him for the match, which Brazil won 2-0, because the request came after a FIFA deadline.
"It might be something he thinks about in his free time, but on the field he is the same as always," Ranieri told reporters. "He is having an excellent championship and he always plays for the team.
"He isn't distracted. This lad has fought all his life to get where he is at Juventus."
Amauri should start alongside Vincenzo Iaquinta up front on Tuesday and should be fresh as he was rested for most of second-placed Juve's 1-0 league win over Napoli on Saturday.
Lazio's Argentine forward Mauro Zarate, who scored the goals that gave his side a 2-0 victory over Bologna at the weekend, is suspended.
Coach Jose Mourinho is likely to rotate the squad of Serie A leaders Inter Milan, defeated finalists in the last two seasons, for Wednesday's visit to Sampdoria in the competition.
