Fields, 22, had just returned to full practice Wednesday after missing the last two games due to an ankle injury.
Bears coach Matt Nagy indicated he would start on Sunday, but under revamped COVID-19 rules, Fields will have to "test out" provided he is vaccinated. If he's unvaccinated and tested positive, he can't return for a minimum of five days.
Fields has started 10 games this season. The Bears are 2-8 with him at the helm.