The Uruguayan striker scored two goals in two minutes, thanks largely to Liverpool’s goalkeeper Jerzy Dudek, to help secure a famous 2-1 victory for the Red Devils at the home of the fiercest rivals.
Forlan’s name is still sung proudly by United fans almost 14 years(!) after that sunny December afternoon, despite the fact he mostly struggled during his two-year spell in England.
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The striker, of course, went on to enjoy a fantastic career, flourishing in Spain with Villarreal and Atletico Madrid while winning the prestigious Golden Ball award at the 2010 World Cup finals.
Forlan turned 37 in May but he agreed to sign for the Indian Super League side Mumbai City FC earlier this month.
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Forlan: United can sign me on loan!
The South American admitted he had received offers from teams in England, but revealed in The National why he had to turn them down.
“I had offers to play in England this summer and last. The only team I will play for in England is Manchester United,” he wrote.
“If Zlatan Ibrahimovic, Wayne Rooney, Anthony Martial and Marcus Rashford all were out of the picture, I would be happy to go on an emergency loan and help out United.”
Forlan: I'd score against Liverpool again
Forlan, in a comment that will endear him to the United fans even more, continued: “Maybe play at Liverpool away, score a goal or two and then make way when the injured come back.”
He added: “I had offers from Spain and Italy, too. But I feel that I have done my time in both countries, sampled the life there and enjoyed myself. Now I am ready for something new.
“India is definitely that. Cricket is the No 1 sport, especially in Mumbai where I am headed, but the Indian Super League has been a success.”
Forlan isn't ready to retire just yet
Forlan also has no plans to hang up his boots following his spell with Mumbai.
“Over a billion people live in India, so the potential is huge,” the former United forward said. “The organisers want to raise standards because India have not done well in football.
“I have been to India once, to the football city of Kolkata, but it was a brief visit. I have heard people say that India is a great place, others say it is a terrible place. I am fortunate to have the chance to find out for myself, to form my own opinion. If I like it, maybe I will go back the following season. Who knows?
“I will be 38 then, but Ryan Giggs and Teddy Sheringham played until they were 40. I’d love to play until the same age.”
Did Manchester United give up on Diego Forlan too soon? Have your say by leaving a comment below.