The United States' remarkable 3-2 victory over Group D favourites Portugal has done nothing for Poland's chances of getting to the second round.
Portugal underestimated the USA and paid the price," striker Cezary Kucharski said.
"Now we know it's not going to be easy to win against any of the teams."
After losing to co-hosts South Korea in their opening game, the Poles know that anything less than victory against Portugal on 10 June will leave them on the brink of elimination.
Coach Jerzy Engel said: "So far, no team has played such 'athletic' football as the USA in these finals. "It was impressive that they ran so much.
"For us, Portugal's defeat is the worst that could happen. But Group D has teams on a similar level and anything can happen in the next games.
"I think we are capable of beating both the Portuguese and USA teams.
Defender Jacek Bak was also upbeat.
"I never thought that such result was possible. I was in shock when the USA added the third goal.
"They are very well prepared for the finals, but we are too. It's just a pity we didn't show it in Busan."
Bak and Jacek Krzynowek trained separately from the rest of the squad on Thursday, with Krzynowek complaining about his ankle.
Maciej Zurawski and Emmanuel Olisadebe are also carrying bruises.