The Bavarians had started reasonably well but Shinji Kagawa opened the scoring in Berlin for the German champions and although Arjen Robben equalised, a penalty from Mats Hummels and a Robert Lewandowski strike right in injury-time at the end of the first-half put two goals between the sides at the break.
The latter struck twice more, either side of an effort from Bayern's Franck Ribery, to seal the double for Dortmund and leave Lahm in shock at just how poorly his side had defended.
“We were the better team for 90 minutes, but we made disastrous errors and kept handing it to our opponents on a plate, a situation Dortmund exploited to the maximum," the Germany international is quoted as saying by his club's official website. "We just can't make errors like that next week [against Chelsea].”
Meanwhile, Dutch winger Robben admitted that he is glad that Bayern still have the Champions League final against the Blues to look forward to after their devastating defeat by Dortmund.
“Losing 5–2 hurts a lot," the Netherlands international admitted. "I can't explain that performance.
"We're frustrated now, but as of Monday our focus is on the Champions League final.”
Bayern will be bidding to claim their fifth European Cup title when they host the Blues at the Allianz Arena next Saturday at 20.45CET.