In my view, giving someone a negative review is only one among many options for dealing with a performance issue. You could always give a neutral review (in terms of the overall rating) and discuss the issue with them verbally, or pledge to resolve the issue in the next year.

If it means you have to let someone have a neutral review that they don't deserve... swallow hard, and start managing after the review is done.

Remember that your employees will be interpreting the review in light of the corporate culture, which they are fully aware of. They will also know whether the problem that you discovered is real or not.