Carragher believes United should have found an extra body in midfield.
He told the Daily Mail: "When United fell behind to Pedro's goal, a clear pattern of play had been established but it would have been very difficult for Ferguson to take off Javier Hernandez, one of his main goal threats, to get an extra body in the middle.
"Once Wayne Rooney restored parity, though, the change could have been made at half-time.
"United needed three in midfield, where they were being overrun.
"An option would have been swapping Hernandez for Nani and moving Park Ji-sung inside; alternatively, Darren Fletcher could have come on, allowing Carrick to drop deeper, as Busquets plays for Barca.
"Two of the goals Barcelona scored came from them playing in between the lines where a holding midfielder would have been but, then again, I think whatever changes were made, the outcome would have been the same.
"Gerard Pique showed why he is one of the best defenders in the world, Xavi the greatest midfielder of all time and Lionel Messi displayed the form that has made him the greatest player of all time."