Portugal international Deco believes Barcelona have the ability to score a goal at Old Trafford and deny Manchester United a place in next month's Champions League Final.
United start as favourites to clinch a place in the Moscow final after drawing 0-0 in the Camp Nou last week, but Deco sees the situation differently.
"The fear is with them now," he said. "They know if they concede at home they will have to score twice to win the semi-final. I'm optimistic.
"In the first leg Messi had a lot of players on top of him when he received the ball, but he still caused problems.
"Now they have to attack us more and they will not be able to concentrate so much on covering Messi.
"When I saw that Tevez, Rooney and Ronaldo were all starting on Wednesday, I thought they would pressure us high up the pitch. But they defended quite deep and can't do that at home."