Tullberg admits he urged his players to have fun going into the Europa League tie at the City Ground.
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He said after the famous win: "When you win away against a Premier League team from England as a Danish player, you are happy, but I'm also happy with the performance.
"I'm happy for the boys and the club. The guys are working hard, and they deserve these Europa League games. I told them to go out and remember being the small child; why we are playing football.
"It's important we leave everything out there, and we did that."