Nunez became Benfica's most expensive signing when he arrived from Almeria in a €24million deal in September, and he has already caught the attention of LaLiga powerhouse Barca.
The 21-year-old forward opened his account with a hat-trick in the Europa League last month, while the Uruguay international has netted five goals across all competitions.
Juventus have also been linked to Nunez and when asked about the Benfica forward's future, Lasalvia said via Sport: "Darwin will remain at Benfica at least until 2022."
Benfica head coach Jorge Jesus has tipped Nunez to surpass Joao Felix as the club's most expensive sale.
Joao Felix left Benfica for Atletico Madrid for a club-record €126m last year.
"We got it right. We must not forget that he is a kid, he is 21 years old, there is a lot about the game that he does not know yet. He will learn with me and Benfica," Jesus told SportTV.
"All this decision-making ability, finishing and speed… He was Benfica's most expensive purchase, and when there is no pandemic, he will be the most expensive sale. He will be a world-class player.
"Unfortunately for me, maybe I will lose him in a short time."
Benfica suffered back-to-back Primeira Liga defeats following Sunday's surprise 3-2 loss at home to Braga.
The result has left Benfica four points adrift of leaders and rivals Sporting CP.