The deal will see both players remain at the City Ground for the 2014/15 season as part of a loan arrangement between the two clubs, but the Reds boss insists the highly-rated pair have been allowed to leave against his advice.
Central defender Lascelles, 20, and goalkeeper Darlow, 23, have been tracked by several Barclays Premier League clubs but the Magpies have won the race to secure their signatures - reportedly paying Forest in the region of £5million.
Karl Darlow (PA Sport)
Pearce, who spent two years at Newcastle towards the end of his playing career, only last week insisted that none of his young stars would be sold unless it was right for the club.
On Monday evening he told the Nottingham Post: "We had an offer from Newcastle, that I was made aware of last week, in regards to the two players going there.
"The figures that were mentioned, by the representative of our club - I said I would not do that deal, when I was asked the question.
"I thought I could push their value up and that they would be worth more in the long run.
"The first I heard today about things was when Jamaal Lascelles came in and told me that he had a medical at 2pm at Newcastle.
"That disappointed me greatly."
Academy product Lascelles has been on the radar of the country's big clubs for a few years now. Forest turned down an offer from Arsenal in 2011, while in January this year Everton were fined £45,000 and warned as to their future conduct after making an unauthorised approach for the player in 2010.
More recently, Tottenham have been linked with a move for the England Under-21 international who became a first-team regular for the Reds last season.
Darlow joined Forest in 2006 after being released by Aston Villa's academy as a teenager. Some impressive displays last season saw Premier League scouts flock to the City Ground to watch him, with Liverpool and Spurs thought to be particularly keen.
He emerged as the club's number one back in 2012 and has gone on to make 67 first-team appearances.
The acquisition of Lascelles and Darlow takes Newcastle manager Alan Pardew's number of new signings up to nine, with the Forest duo following Facundo Ferreyra, Siem de Jong, Daryl Janmaat, Emmanual Riviere, Jack Colback, Ayoze Perez and Remy Cabella to St James' Park.
- Sports & Recreation
- Barclays Premier League
- Jamaal Lascelles