Sonko has just played a full 90 mins of reserve team football at Derby.
"I drove up to Derby on Monday to see the reserves," Coppell told the club's official website. "Sonko got 90 minutes, his first competitive 90 minutes.
"He was up against Robert Earnshaw and Jon Macken, millions of pounds' worth of strikers, and did a really good job.
"He looked tired in the dressing room afterwards, a bit weary. He's on course, that's all you can say.
"Would I feel comfortable playing him at the weekend? Probably not. I could do it, I know I could, but it's probably best to give him longer.
"After this game we've got the two-week break. There's one reserve game scheduled in there, and we'll try to add one more.
"Then he will have had two more 90 minutes under his belt, and he'll be ready."