The Foxes fought back through Jamie Vardy to cancel out Ryan Yates' opener for the visitors. However, Forest took charge after the break thanks to a brace from Chris Wood.
Cooper said of Pereira's display, “He’s a strong option that we have in that position and Ricardo did some good things no doubt and the change was nothing on his performance, it was just on trying to make some changes in the game.
“Obviously they put some defenders on so we just wanted to get some more attackers on and try and create some chances but unfortunately that wasn’t the case.”
The 44-year-old added: “It’s going to hurt for sure and the disappointment will be even bigger because we’ll reflect and know that we should have done better. As hurtful as it is, we have to move on as quickly as possible.
“We have to pick the bones out of this, but we’ve got to look forward and move forward. There’s going to be ups and downs and this is definitely a down. We don’t want to be losing games like this in the way that we did. We have to put it right as soon as we can.”