Forest were thumped 4-0 by the Foxes on Monday night.
Cooper said: "It is another disappointing result, there's no doubt. We weren't good enough tactically in the first half. We came with a plan and we were nowhere near good enough in the duels which was disappointing.
"We should have gone 1-0 up, we had the chance of the game up until that point. But we are not getting good results at critical moments and again we had a spell that took the game away from us.
"I thought we did better second half, and again we had another 1v1 where we could do better and maybe the game would have changed a bit. But we are not taking those critical moments and we are at a level where it really matters
"Without sounding too soft it is really difficult to be too critical of the group because it is a completely new group and we are finding out about ourselves, the job.
"The players never gave up. I'm not questioning their effort or work rate. There were times where I felt we could have been more competitive but I'm not looking at these lads and thinking they don't care. They are just a really fresh group trying to come together."