After earning a draw with Germany on Saturday, Ghana remain in contention to advance from Group G and into the knockout phase at the FIFA World Cup for the third tournament in succession.
Meanwhile, Portugal's last-gasp 2-2 draw against the United States on Sunday kept their slim qualification hopes alive and set-up an important clash with Ghana in Brasilia.
With unbeaten pair Germany and the US set to do battle, Ghana must beat Portugal and hope for a favourable result in Recife.
And Appiah believes there is a lot more to Portugal than just nullifying Ronaldo's influence.
"He's the best player in the whole world - everyone recognises that," he said.
"But it's about teamwork, I don't believe in the individual.
"It's important that we don't allow him to play his game. We will study their game, but it's not only him, it's the whole team.
"It won't be easy to reach the Round of 16 because we've only got one point. It'll be tough, it is possible.
"You can only stop Cristiano Ronaldo, the best player in the world, by working as a team. We can't let him show what he can do."
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