Robert Page’s side will face the winner of the play-off semi-final between Ukraine and Scotland in Cardiff on Sunday to decide who takes the last European qualifying spot at this year’s World Cup in Qatar, and their key players were rested for the match in Wroclaw as a result.
With Gareth Bale among the players left out, Daniel James of Leeds United was the only player to start for Wales who also started in the 2-1 win over Austria in their World Cup play-off semi-final in March.
Jonny Williams of Swindon Town put Wales in front early in the second half in the Nations League A, Group 4 match, but a Poland side captained by Robert Lewandowski fought back to take the three points.
Two substitutes got their goals, with youngster Jakub Kaminski — who will move to German Bundesliga side Wolfsburg for next season from Lech Poznan — equalising in the 72nd minute.
Fellow substitute Karol Swiderski, who plays his club football in MLS, then struck Poland’s winner five minutes from time.
Belgium and the Netherlands complete Group 4 and those two sides will meet in Brussels on Friday.
After their play-off, Wales are due to host Belgium on June 8 and the Netherlands three days later.
Wednesday’s match was the very first in the new edition of the Nations League, which was won last year by France.
The action continues on Thursday when Spain host Portugal in Seville in the standout game.