Guus Hiddink's side travel to the Nou Camp on Tuesday night for their first-leg encounter with Pep Guardiola'a la Liga high-fliers without suspended left-back Ashley Cole, meaning right-sided defender Jose Bosingwa will be tasked with trying to stop 'The Flea' in his tracks.
However, Ferguson - whose United side will face the winners of the tie in the final should they emerge victorious from their last four clash with Arsenal - believes achieving it for the entire 90-minutes is practically impossible if the diminutive Argentine is on form.
"Trying to stop players like Messi is quite a challenge," he said in the May issue of The Technician, the UEFA newsletter for coaches.
"There are always moments in a game when he drives at you with the ball and you say: 'Oh no, here we go again.'
"All the great players, like Lionel Messi, have an element of courage that elevates them above everyone else - this is apart from their outstanding ability."
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