Leicester City have already met the buyout clause in Campbell's contract.
"He's scored some crucial goals, so obviously he's a very good player," said Earnshaw to BBC Sport.
"Fraizer scored some goals and Cardiff are a good team to play at, so he leaves his print on the team.
"He's done very well in all his time here, he puts the effort in which is one thing you have to have.
"It's going to be difficult."
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