The Roma midfielder helped the Italians to get off to a winning start against England in Group D but they now face a dangerous Central American side who also took maximum points in their opener.
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"We should fear them a bit. You need a healthy fear against a team that surprised everyone with their victory over Uruguay," he told the press.
"Nobody thought they would get such a clear result against a very strong side.
"They'll be fired up after that win. If they had lost 4-0 then we could've underestimated them but, in this case, we cannot possibly do that.
"I hope and am convinced that we can virtually secure qualification by beating Costa Rica. A victory would put a strong stamp on our ticket to the next round."
Italy hope to welcome back captain Gianluigi Buffon from injury for the match, while De Rossi himself should be fully fit after sitting out of training with a neck problem.