The 23-year-old left-back has agreed a two-year contract with the club and says that he's looking forward to the new challenge.
Speaking to Sky Sport News he said: "I'm nervous because it's happened all of a sudden and there's a big culture change. But, there's excitement there as well and I'm looking forward to it."
Lech have qualified for European competition next season, having guaranteed the runners-up spot in the Polish Ekstraklasa division, and Douglas said it's a move he couldn't refuse.
He added: "There's more lows than highs in football and I don't want to live with any regrets which is why I think I have taken this challenge as well.
"Part of the deal is that me and my partner go to Polish lessons.
"Obviously that's the first language here and I am sure I will pick up little bits as I go."