Bayer Leverkusen face second-division Kaiserslautern in Saturday's German Cup final licking their wounds after a first loss in almost a year.

Xabi Alonso's side were thoroughly outclassed in a 3-0 loss to Atalanta in the Europa League final on Wednesday, their first defeat in any competition since May 2023 -- a run of 51 games.

Energetic and dynamic this season with a taste for late goals, Leverkusen looked a shadow of the team which thundered past Bayern Munich to win their first ever Bundesliga title this season.

In a year when Alonso pledged to stick around with the club despite reported interest from Liverpool and Bayern, Leverkusen looked on track for a remarkable three trophies.

Now they need to rebound to stop their underdog opponents, who flirted with relegation to the third division for much of the campaign, spoiling what has been an incredible season.

A loss will recall the club's infamous 'Neverkusen' season in 2002, when they somehow emerged empty-handed despite being five points clear in the league with three games remaining, while also qualifying for the German Cup and Champions League finals.

Leverkusen's unbeaten run, which broke a 59-year-old record set by Portuguese giants Benfica in 1965, was bound to end sometime but it was the manner in which the club capitulated which will concern Alonso the most.

The Germans were outmuscled by a determined Atalanta, with the reliable Florian Wirtz and Granit Xhaka, so important this season, having off nights.

Winger Jonas Hofmann summed up the club's struggles on Wednesday night, lamenting "many mistakes, individual mistakes, bad passes and simple technical errors," saying the performance was "not Bayer-like".

"We weren't confident in the build-up," Hofmann said, explaining Leverkusen had given Atalanta the feeling "they could stand up to us and win the title".

- 'Transform pain' -

For a side which has made a habit of scoring late goals -- they have scored 15 times after the 90-minute mark this season -- Leverkusen were not able to turn the tide.

Alonso was self-critical after the match, saying "we weren't at our best, including me" and explained the system he chose "did not work," urging his side to "transform our pain into positive energy".

Despite the disappointment, beating Kaiserslautern would make Leverkusen just the sixth side in German football history to do the double -- an incredible achievement for a team which had two major trophies to their name before this season started.

Xhaka told reporters on Wednesday the result was "not easy" but "we will not let it destroy us".

"If we don't win the triple, then we will win the double."

Centre-back Jonathan Tah, captain on Wednesday night, said "the show must go on".

"We've got a very important game ahead of us -- we can't afford to talk ourselves down too much."

Their opponents may now face a wounded Leverkusen hoping to make amends, but also understand Alonso's side are vulnerable after Wednesday's surprise loss.

Despite their run to the final, Kaiserslautern only played one top-division side in the Cup -- beating Cologne, who were relegated to the second division at the end of the season.

Kaiserslautern striker Ragnar Ache told AFP he had been cheering for Leverkusen in Wednesday's final in the hope Alonso's side "would maybe party a little bit" into the night.

Ache however said Leverkusen "have a very big squad with high quality players -- and they're used to it".

"It's still not going to be easy for us. We have to look at ourselves and what we are going to do."