Ricardo Quaresma struck twice in the first 10 minutes and Jackson Martinez was on target in the second half after Thiago Alcantara had pulled a goal back for the Bundesliga champions in their last-eight first-leg loss.
Bayern captain Lahm warned the Bavarian giants must not dwell on their surprise defeat at Estadio Do Dragao and stressed that all is not lost ahead of the second leg at the Allianz Arena next Tuesday.
He told the club's official website: "The team reacted well to the agony of going two down after 10 minutes. We could easily have lived with losing 2-1, but their third was a bad goal to concede and hurts us a bit.
"It makes it tough, but not impossible. But cursing our luck won't help us now, we have to move on from here. Winning 2-0 or better at home is a possibility.
"Porto have a couple of important players [Alex Sandro and Danilo] suspended for the return. We have to play our football, create chances by controlling possession and put them away. That would give us a very good chance."
Thomas Muller knows Bayern have given themselves a lot to do, but the Germany forward is also optimistic that they can move into the last four.
He said: "It was a start to forget. We were 2-0 down after 10 minutes without our opponents creating a single chance from open play. Our reaction wasn't bad.
"We stayed calm, we dominated the game in the first half and pulled a goal back. It's a shame the game became so scrappy in the second half, and we weren't able to play our football any more.
"Porto did a good job of parking the bus. Then we've gone and conceded another silly goal, but we can still go through. It wouldn't be the first time we’ve won a match at home.
"We're optimistic, but we know we have a lot to do. We've not made it easier for ourselves."
