His goal came during a period of domination in the 26th minute as Valarengen defender Tom Henning Hovi was adjudged to have tugged his shirt. The penalty was dispatched low to the goalkeeper's right with ease.
England's World Cup missing quintet of David Beckham, Rio Ferdinand, Paul Scholes, Nicky Butt and Wes Brown gave the likes of Forlan, Phil Neville, John O'Shea and Danny Pugh an opportunity to impress the boss.
The lively Forlan took his chance and he capped a good display with a delicate cross that allowed Roy Keane to poke in the winner with five minutes remaining after Pa-Modou Kah had equalised with a powerful drive in the 75th minute.
Juan Sebastian Veron's influence on the game was also eye- catching and his passing was intelligent, courageous and measured.
Ferguson, who was impressed by the reception from Norwegian fans and by the standard of opposition, said: 'Forlan was the man of the match and he gives us options in the middle and on the right. He is settling in, he's just 22 and he's going to be a good acquisition.'
