Disgruntled Forest manager Stuart Pearce risked the wrath of chairman Al-Hasawi on Monday when he revealed that the highly-rated duo had left against his wishes.
Pearce described the deal, for a combined fee reportedly in the region of £7million, as "not a great business decision" less than a week before Forest's first game of the Championship season against Blackpool on Saturday.
However, Al-Hasawi released a statement on Tuesday revealing that goalkeeper Darlow and centre-back Lascelles, who will spend this season on loan at Forest before moving to the Premier League club permanently, were sold in order for Forest to comply with Financial Fair Play.
The Kuwaiti businessman said: "The manager and I openly discussed the proposed sale of Karl and Jamaal and he was aware of the negotiations throughout. The business side of football is changing and the requirement to increase revenue means, unfortunately, difficult decisions have to be made.
"As Stuart mentioned in his media interview, I have always backed him financially and that will continue. I respect him greatly and I strongly believe together we can achieve our aims and bring success back to this great club.
"Should the pair's proposed move to Newcastle be completed as expected, the transfer fees will significantly help us meet the stringent Financial Fair Play criteria, all the while allowing us to retain two high-quality players for the 2014-15 season, paying nothing towards their wages.
"Sadly for me, there is no room for sentimentality in the running of a football club and it is up to me to start making some tough decisions to preserve the long-term sustainability of Nottingham Forest.
"But while balancing the books is a hugely significant part of my role, it remains my job to ensure the manager has the best possible squad to work with as we pursue a place in the Premier League.
"Rest assured, therefore, that more additions will be made and the squad will be trimmed further to ensure we have a balanced and talented group of players in place."