Messi missed Saturday's laboured 1-0 win against Levante and two games before that for the Spanish league champions-elect since coming on to help inspire his side in their quarter-final second-leg with Paris Saint-Germain.
"He's very important for our attack and you could see we missed him in our last few games," said Xavi.
"I am sure he will be at his best so we can again see what an awesome player he is. It's important to have him in our ranks and it will have an influence on Bayern Munich if he plays.
Barcelona are competing in a record sixth successive Champions League semi-final and are looking to lift the trophy for the third time in five seasons after their successes in 2009 and 2011.
"We'll attack, we don't know how to play any other way and we need away goals, this is a strong opponent, but I think we are coming to Munich at the right time," Xavi continued.
"Ball possession will be crucial, we know we are at our strongest when we keep the ball.
"It's a 50:50 match, they are German champions, in their cup final and have reached the semi-finals without problems, which couldn't always be said for us.
"The Milan game was the reference for us, we held the ball very well, moved around and we had a lot of pressure from the back.
"We were playing completely on attack, the Milan match was the most important for us, more than the PSG game."
"We will try and dominate the game so we can take a lead back to Barcelona."