Marc-Antoine Fortune ensured there would be no way back for the hosts, but it was Powell who earned special praise from his manager.
"Nick is a player that excites me because he can play everywhere," said Rosler, whose side sealed a home tie against Crystal Palace later this month.
"At Man United he was one of the midfield players - today he came on as one of the midfielders, then I put him up front and in many games I've played him from a wider position coming in.
"I can also play him when I need him as a number nine because he has a fantastic physique and technique.
"Every time he plays for me he gives me 110 per cent and he was fantastic tonight.
"We gave a solid, hard-working performance, we raised our levels throughout the game and we took over more and more.
"I thought the game started very open and we had some great chances and I couldn't believe it when they went 1-0 up.
"But the longer the game went, the more confident we got and obviously the substitution in bringing Nick on turned the tie in our favour."