The Championship side are now one point above the relegation zone after a 5-1 thrashing by Burnley at Turfmoor last night (Tuesday).
"My assessment? Rubbish,” Sky Sports News quotes McClaren as saying. "Unacceptable for the fans who travelled.
“We looked far too vulnerable. I didn't see it coming but it seems to be the story of the season - one step forward and three steps back. It's hurting us.
“It was rubbish but our problems are not just tonight, they're about stopping goals all season.”
The former England manager hinted that he is unaware of how long he will be manager at the City Ground.
"It's for other people to decide about my job but it's not about me or my job, it's about the team and as normal after a defeat like that, we ask for a reaction,” he added.
Forest will try to improve on their Championship record with a home match against former Premier League side Birmingham City on Sunday.