The 18-year-old striker was called up by Lambert to train the first-team this week.
And the Scotsman thinks Campbell has plenty of potential.
"He's a definite goal scorer," said Lambert. "He can be a little bit better in the work aspect, which I've spoken to him about, but the boy can score.
"He's got a big future, without wanting to put a burden on his shoulders, if he listens and works hard. Nobody is going to give him anything, it's not a gift. If he earns the right I will take a look but he has to give me problems. He is definitely one to keep an eye on."