The 24-year-old keeper and the central defender, 21, were signed by the Magpies from Nottingham Forest in a joint deal for an undisclosed fee in August last year before immediately being loaned back to the Sky Bet Championship club for the season.
Then Forest boss Pearce was less than happy with chairman Fawaz Al-Hasawi over the deal, which he claims was done behind his back, and the two eventually parted ways in February.
Former Newcastle defender Pearce believes the club invested in potential, but potential which may take some time to come to fruition.
Pearce told the Journal: "Well, they are both outstanding lads, without question. Their character is first-class. However, I don't think they are ready yet to go straight into the Premier League.
"Both the club and the fans will have to give them time and encouragement. They are potential rather than the finished article.
"Darlow more or less played in my first team regularly, but Lascelles not quite so much. He will require a bit more time.
"However, from their point of view the move is ideal. They weren't about to play for a Premier League club staying at Forest, but now hopefully they are.
"Though I fell out with the chairman over the deal, I told both lads they were going to a fantastic football club with passionate fans. I knew that from first-hand experience."
However, Pearce believes Forest may still regret allowing the pair to leave when they did.
He said: "I had been asked my opinion and I said they should stay for the moment. However, the chairman sold them behind my back. It was his club and his money, so he could do that. I just felt if we had kept them their value would have risen given time."