Their derby draw on Saturday against Espanyol made it four matches without a win for the Catalans, their worst sequence in five years.
They will be without Brazil star Ronaldinho, who is out for the rest of the season with a leg injury, and who has been linked with a summer move to AC Milan.
However, right-back Gianluca Zambrotta believes the return of Messi could spark a tremendous end to their campaign.
"It is very important he is back against United because the whole season is at stake for us in this game," he said.
"We are having problems but I think everything is going to be different on Wednesday."
If we beat them with Messi they will have no excuses
And Barca's former Chelsea forward Eidur Gudjohnsen says the attacking approach of Sir Alex Ferguson's side could play into their opposition's hands.
"United are a team that always want to play football and leave more space for the opposition than Liverpool or Chelsea," he explained.
Meanwhile, Messi declared himself "ready" to face Manchester United after the game against Espanyol and is determined to make up for missing the victory over Arsenal in the Champions League final in 2006.
"The Champions League motivates me more than ever. I could play no part in Paris and that still hurts," he said.
"I have unfinished business with this competition and I want to win it."
Tevez, however, is happy to face his compatriot and says beating Barcelona will carry more credence with him in the team.
"While he is a big danger for us, it is better he plays because we will have beaten the real Barcelona or gone out to the real team," he added.
"If we beat them with Messi they will have no excuses."
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