Jordan Smith said the players need to bounce back quickly after a disappointing defeat against Sunderland on Saturday.
Aiden McGeady's goal was enough for the visitors to take the points from The City Ground and Smith said The Reds didn't do enough.
Speaking after the game, he said: "It was frustrating and we just seemed to pick up where we left off on Boxing Day which is really disappointing.
"All we spoke about was getting a reaction and that we owed it to the fans, then we just seemed very lethargic, lacklustre and we didn't look like we were going to create much.
"I can't put my finger on it; we are really together as a squad and have a game plan going into matches but the execution just isn't there at the minute.
"The last two games have been unacceptable from us as a squad. We have had two home games and we have said how we want to make this a fortress but, with all due respect, we have been beaten by two sides lower than us in the table and we should be beating them.
"It is completely down to us to get out of this; we have got to take a long, hard look at ourselves and do it quickly as there is another game coming up."
Forest travel to Leeds United on New Year's Day with a quick turnaround, and Smith is well aware of the magnitude of the fixture.
"We are expecting a big crowd there," he added. "Leeds are known to support in their numbers so we need to get on top of that and quieten them down quickly.
"It is a big game as it is Leeds vs Nottingham Forest which has historical elements to it. It is on Sky, on New Year's Day so it is a big game whichever way you shape it. But there are no easy games and you just have to deal with it.
"We need points on the board but it is a chance for us to show what we can do."