Barca beat the Red Devils 3-1 in the 2011 Champions League final at Wembley, and Ferguson admitted in his autobiography that Pep Guardiola's men were the finest side he played against.
"Barca was the best team ever to line up against my Manchester United sides," Ferguson wrote.
"The group of world-beaters who formed around Lionel Messi were formidable.
"I felt no envy towards those great sides. Regrets yes, when we lost to them, but jealousy, no."
Barca's 2011 side recorded a historic Treble, winning La Liga, the Copa del Rey and the Champions League.