The Uruguay international endured a rocky three-and-a-half years at Old Trafford before leaving for Villarreal in 2004.
He won the Premier League and FA Cup with the club but his brace against the Reds on their own turf was the highlight of his time in England.
And Forlan has admitted that fans still quiz him about his Merseyside heroics 15 years on.
Diego Forlan scored twice as Manchester United stunned Liverpool in 2002 Jose Mourinho baffled by Man United's Diego Forlan chant Mon, April 10, 2017
Jose Mourinho looked absolutely baffled when Manchester United fans started the Diego Forlan chant during their win over Sunderland yesterday…
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Jose Mourinho got a bit confused when United fans sang the Diego Forlan chant
The 37-year-old, whose name was sung as United beat Sunderland 3-0 at the weekend, reckons it is talked about even more than his performances for Uruguay at the 2010 World Cup.
"Soon after the match ended, I received messages which showed television footage of Jose Mourinho turning to Michael Carrick to ask who Diego was," he told The National.
"United fans were singing about me in the away end, something which I cannot believe still happens.
Diego Forlan's name was etched into Manchester United folklore
"I loved my time at United and had a good relationship with supporters, but I am sure there are many others who say exactly the same, some of them who had more successful careers at the club than I had. And they do not get their names sung.
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"It is all very flattering, but I love that catchy little song: "Diego, wooah, Diego, wooah, he came from Uruguay, he made the Scousers cry".
"People still come up to me wherever I am in the world and remind me of two goals scored in a Premier League match 15 years ago more often than they do about my goals which saw me as the top scorer in the 2010 World Cup."