Forest skipper Ryan Yates, Arnaud Kalimuendo and Nikola Milenkovic struck the goals for Forest as they won 3-0 on Thursday night.
Mravac said afterwards: "We were saying before the game that we needed to do the fundamentals when it comes to running and duelling at the highest level, and we didn't get close to that.
"We defended quite well, but when it comes to set-pieces we were really poor. They scored three goals from set-pieces, which is a big part of football. We didn't deal with it well enough.
"We had, I think, one shot on goal and that was not good enough when it comes to our own game with the ball. To have the possibility to create a good result against these kinds of opponents, you need to do everything - every part of the game - at the highest level.
"We were not close. The most positive thing was our fans and the support they gave us. They were amazing and deserved more from us."