Elliot Anderson and Chris Wood had Forest 2-0 ahead at Spurs inside the first 16 minutes. The hosts didn't reply until on 87 minutes through Richarlison.
Kulusevski remarked after the 1-2 defeat: "Yes sloppy start, we concede three goals in ten minutes. Yes one was offside but then after that it is difficult to get back in.
"(Slow start) can depend on a lot of things. We also played on Thursday night, but you have got to be ready from the start. You come back from a long trip at five in the morning but if you are not on it then you will pay. That's what we did.
"We played some decent football today and created some good chances. They are a very difficult team to play because they are very good at defending and play a lot of men behind the ball. We created a lot of chances. Their goalkeeper made some good saves. I hate to lose. It is a bad feeling.
Mixed emotions
"I am happy to be back. I'm back for the most important part of the season which is important. I feel very good physically. I am very angry that we lost but I've got to think about the process and stay focused.
"When you are in this part of the season and have a lot of injuries you have to take care of that. You have to do what you can do to make people healthy."