Di Stefano died on Monday afternoon after suffering a heart attack on Saturday, the day after he turned 88, resulting in him being placed in an induced coma.
Among the many tributes, Madrid forward Cristiano Ronaldo hailed Di Stefano as a "maestro" while FIFA president Sepp Blatter described the Argentina-born forward as "the most complete player".
Di Stefano won five successive European Cups at Real Madrid between 1956 and 1960, scoring in all five fixtures - including a hat-trick in-front of a watching Ferguson at Hampden Park in 1960.
He sits second on Real Madrid's all-time goalscoring list behind Raul and was an honorary president at the Bernabeu, with Ferguson ranking him among the very best.
"The great question that always comes up is who are the greatest players," Ferguson said to/ Sky Sports News.
"Obviously there is a list of great ones like (Johan) Cruyff, (Diego) Maradona and Pele and of course (Ferenc) Puskas and of course Di Stefano - I thought he was one of the greatest in my mind.
"He had a fantastic balance and poise. If you look at one of the goals he scored in the final against Eintracht Frankfurt it was completely what we are saying about the balance and poise, unbelievable he was.
"I was very sad to hear the news. Even at 88 it is a bit of a shock. I saw him two years ago before our Real Madrid game and he looked fine and looked full of health so it is disappointing to hear the news."
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