The Germany international hopes the Argentina star will not feature on Tuesday and believes the Catalans are favourites
The Bavarians have made it to the final of the competition twice in the past three seasons, yet Barca have done even better as they emerged victorious three times since 2005-06.
"Of course they are weaker without [Lionel] Messi, we saw that during the PSG game. We're hoping he doesn't play tomorow," Lahm said at a press conference.
"Barcelona have been the team to beat in the last few years and we have to recognise this."
The right-back is hopeful Bayern can get a good result in the first leg, though.
I think we stand a good chance of winning and our aim is to do that tomorrow. We have a great bunch of players who know what it takes to win.
"It will be important to get a positive result at home.
"I'm delighted to have reached the semi-finals. Now it's up to us to go one step further and make the final. It doesn't matter about the quality of the opposition."
Tuesday's match at the Allianz Arena is scheduled to kick off at 20:45CET, with the return at Camp Nou taking place next week.