Ex-Reds striker Forlan, the scorer of four goals for Uruguay at South Africa 2010, admits he learned a great deal under the United manager's tutelage.
"I have a lot of memories of Sir Alex, he spent a lot of time with me and he taught me a lot of things," Forlan told ManUtd.com.
"The way he treated from the first time I arrived in Manchester was great. He was waiting for me in the stadium and he took me to the hotel and showed me all the facilities at the training ground - I really appreciated that. He is a very nice person."
Forlan was just 23 when United signed him from Argentine side Independiente in 2002; despite mixed fortunes in England, the Uruguayan has no regrets about his Reds career.
"I'm not sad at the way it ended for me at United," Diego told us. "I was really happy during my time there and thank the people at the club for the way they treated me. I learnt a lot of things and had a good relationship with everyone.
"The reason I left was because I didn't have the chance to play as regularly as I wanted at the time. There is nothing to regret or to say to anybody. I accepted it and I can now play regularly, just as I wanted, and I am really happy I'm doing that."