The German giants went down 4-1 on Wednesday night after Brazilian winger Raphinha scored a stunning hat-trick to leave Bayern with just three points in three games in the competition.
And now, Germany and Bayern legend Matthaus has added fuel to the fire as he took aim at the performance of Manuel Neuer.
Neuer struggled on Wednesday, conceding four goals despite Barcelona only racking up an xG of 2.23.
Lothar Matthaus has ripped into one Bayern Munich star after they were thrashed by Barcelona
The 38-year-old has been integral for Bayern ever since joining in 2011, making more than 500 appearances and winning 28 trophies along the way.
However, in recent years, mistakes have become increasingly common and Matthaus felt the goalkeeper was no longer at the level he once was.
'I don't normally want to criticise Manuel,' he told BILD. 'But at the moment he is not the support for the team that he was in the past.
'Manuel Neuer is no longer Manuel Neuer. Because he doesn't save the unstoppable balls, his goalkeeping has changed.
'He used to anticipate every situation, he had a 360-degree view, and he would initiate attacks with a build-up pass. At the moment he is not giving the defence any security.
'This is also demonstrated by the rate of just over 40 percent of shots saved in the Bundesliga. I hope that he will soon find his way back to his old game and confidence.'
Neuer has not always been a regular for Bayern since breaking his leg in a skiing accident in December 2022 and the 38-year-old has come under pressure for his position.
And, Matthaus' criticism of Neuer - who retired from international duty in the summer - was joined by several other pundits, with former Bayern star Markus Babbel claiming that Neuer 'played as if he had ski boots on'.
Meanwhile, former Bayern and Fulham boss Felix Magath added: 'I'm a little worried. Because he didn't make a very good impression today and I'm wondering what the reason is.
Matthaus went onto insist that Neuer can still return to his best, but said he needs to find a way to build up his confidence.
'At the moment, he is also lacking luck in the game,' he added. 'But when he had eight dangerous shots on goal in the past, as in (the games against) Frankfurt and Barcelona, this did not result in seven goals being conceded.
'The second goal against Barcelona is a good example: Manuel hesitated, then left the goal and was unable to do anything about the goal.'