Forest won 1-0 at the City Ground through Anthony Elanga's first-half goal.
Postecoglou later said, "It was a tight game obviously, with the way that Forest played it was never going to be an open game. We had some good chances to get a goal but we needed to gain momentum in the game. Unfortunately, we couldn't get it and they scored from their counter attack. Unfortunately, we were unable to claw it back.
"You know when the game is in the balance you don't want them to gain the ascendency. I thought we were in control more at times. When they scored, I thought we had the better chances. A disappointing goal but we had good chances after that to get back in the game.
On what he felt was missing in attack, he added: "Just a combination of things. There's not a lot of space and the keeper has pulled off a couple of saves so we've kind of let that opportunity to get a clear cut view of goal but we couldn't get a breakthrough.