He was speaking after Prime Minister Mateusz Morawiecki announced a swathe of tougher rules designed to stem an escalating COVID-19 epidemic.
Poland reported a record rise in infections for the third consecutive day on Friday, confirming 13,632 new cases, the most since the pandemic hit the country in early March.
Niedzielski told reporters: "There is a critical week ahead of us.”
He added: "If this trend continues, next week (...) we will be exposed to the risk of 20,000 to 25,000 cases daily. Such a scenario is worse than a pessimistic one.”
Announcing a stepped-up battle against the epidemic, Morawiecki said that from Saturday children under 16 will be allowed to leave their homes only under the supervision of an adult between 8am and 4pm.
Pubs, restaurants and cafes will for two weeks only be able to provide take-aways and delivery orders.
In a further move, gatherings of more than five people will be forbidden. People who live or work together will be exempt from the rule.