The Lilywhites were soundly beaten at the City Ground, going down to a 3-0 defeat - Lewis Grabban scoring twice either side of a Jack Colback volley.
They very much played second fiddle to Forest, struggling without Alan Browne and Sean Maguire, with Patrick Bauer forced off injured early on.
PNE head coach McAvoy said: "My feeling was one of bitter disappointment. In the first half we didn't do well enough.
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"Losing Browney with a back injury and Seani to a sore Achilles put us into the position where we had to change the team.
"I felt the guys I brought in would do the business for us. Then losing Pat 15 minutes in or so didn't help us much.
"Goals changed games and so do decisions.
"I thought we had a good claim for a penalty when Emil Riis was fouled in the first few minutes and we didn't get it.
"I've seen their penalty back, I can understand why he has given it but I didn't think there was much contact.
"Their second goal was poor defending from our point of view, we headed it clear and then we turned our back on the strike coming from the edge of the box.
"The third goal they managed to keep it alive at the top end of the pitch. We didn't defend it well enough in the first or second phase, it was a scrappy goal.
"I thought all three goals were poor to lose from our point of view.
"At the other end we didn't offer enough, I just felt we looked a bit shot-shy when we got in the final third.
"We found Daniel Johnson in some good pockets, split them and worked the ball into the wing-back areas.
"For whatever reason from there we started to come inside rather than going on the other side and putting the ball into the box.
"That was maybe a wee bit tentative of us.
"We changed it 10 minutes into the second half, went to a back four and three up top.
"If I'm honest I didn't think we created enough, we probably looked less of a threat when we went to a front three which is disappointing.
"We had a lot of the ball but didn't enough with it.
"In the first half we were sloppy with it, didn't match runners well enough and didn't defend well enough as a group."
A key moment came in first-half stoppage-time when Brad Potts blazed a great chance over the bar,
That would have reduced the deficit to 2-1 but the midfielder lacked composure.
McAvoy said: "That was massive and we have been here before because it's happened a few times.
"If that goes in it makes it a different game. There's not a lot I can say about that. He was there, there was some fantastic work from Emil with the cut back.
"You hope that Pottsy can stick it away but he put it into the stand. That can happen.
"I was hoping that after that excellent result the other night that we could see a turning point to kick us on.
"Sometimes it is two steps forward, three steps back and that is what happened today."
McAvoy reported that Bauer's injury was a stretch on the Achilles tendon, the one he ruptured last season.
While not serious, no chances were taken in terms of him playing on and risking more damage.