The former England U21 striker has not scored a single goal in a Potters jersey since arriving from West Brom for £12m a year ago.
"I think he's a terrific finisher, I really do, I think he finishes really naturally," said Lambert.
"He's got a wee bit of work to do on certain aspects, but training wise and finishing, excellent.
"We can only pick 18 guys and we have to get the balance right, and he's not played for a while as well, so he's got to get up to speed.
"What I will say about them is that every one of them has trained brilliantly. They are getting there and Berahino can definitely finish."