The Chelsea midfielder was a team-mate of Messi's at the Nou Camp, and admits the Argentine genius is one of a kind.
'I think Barca have an advantage as they have Messi," said Deco. "He's a player who can make the difference more than all the others.'
Chelsea head towards last four with first leg win over LiverpoolSpeaking about his return to Barcelona, Deco added: 'To return to the Nou Camp is special. Barca is the club I dreamed of playing with. I watch them whenever I can.'
Ashley Cole's first-leg suspension makes Chelsea's task in Spain all the more difficult. Chelsea have Know substitute for Cole's pace and dynamism, and will be forced to play a right-sided defender at left-back.
And Dani Alves, the brilliant right-back who has formed a devastating partnership with Messi this season, admits Barcelona will look to feed Messi whenever possible.
"He is spectacular to play with in a system which allows him such freedom," said Alves.
"He will be the best player in the world without doubt if we can give him the support he needs and he stays on the same path, and he would deserve it.
"What we have to do is help him earn the trophies his talents merit. He loves playing for this team and we are grateful to have him alongside us. He is definitely better to play with than against.
"Our strikers, though, are without equal. We have a very clear idea of what we're going to do. That is the mentality we will go into the semi-final with, because it is goals that will take us through to the final."