Messi will arrive at San Siro on Wednesday for the first leg of their last 16 encounter.
"It will be very difficult to beat Barcelona," the former Italian Prime Minister told RTL 102.5 on Monday morning.
"I still haven't found the time to talk to boss Massimiliano Allegri, but I won't tell him what side to pick - I've never done that.
"I know the role of the President and that of the Coach - I respect those - but a President has the right to give advice to his Coach.
"On this occasion my suggestion would be that Messi is man-marked."