Bopp is trialling with Pompey and Hart told The News: "I brought him from Bayern Munich when manager of Forest nine years ago and he's got everything.
"I can only speak from what I saw in him as a kid at 15-16 and he headed it, was strong, scored goals and was one of the best penalty takers I have ever seen.
"The problem has been he hasn't gone on. I think he's a good player, though.
"It's saying the obvious but he fell away and he knows it. I don't know really what went wrong. He's got a lot so there's something missing. Now I have got to find out what's missing.
"Maybe it's nothing, maybe it's all going to come back. It's a chance to take, though.
"You saw against Havant & Waterlooville there's a little bit of something there.
"Now he's getting a little bit of an opportunity to either stay here or put himself in the shop window."