That's not even close to being true.
I would suggest watching a few behind the scenes featurettes on how they do performance capture for these things and how integral the actor's work is, especially in something like Avatar or the LOTR/Hobbit movies.
My point is, Saldana's performance wasn't critical to the success of the movie, in my opinion. Any half decent actress could have done the same thing she did and the movie still would have been a blockbuster.
It is certainly very possible that it would have been a blockbuster with someone else.
Then again, I'm sure James Cameron auditioned plenty of candidates before hiring Saldana so he must have seen something there that he thought would fit with & elevate his movie.