Critics have already written off the Premier League side's chances of beating the European Cup holders before a ball has even been kicked.
But Messi is taking nothing for granted. He said: "There are no easy games when you reach the knockout stage.
"Our opponents are there on merit and we do not take any threat likely. There is no room for complacency or presumption.
"But they have vast experience and we will treat them with the utmost respect. Games in the past have been very close, there was not much in them."