The striker, who arrived from Olympiacos, stabbed home in stoppage time, and Kelly believes that he will get better once he reaches full fitness.
"It was a great goal, a striker's goal - off his bum, off his hand and into the back of the net from a couple of yards," Kelly told Sky Sports News.
"I'm really pleased for him. He'll get better when he gets to know everybody and gets fitter. He's a goalscorer, a goal poacher and it's very early days for him."
Forest are fifth in the Championship.