Beckenbauer feels Pep Guardiola will need to strengthen during the off-season and make up for the noticeable loss of players such as Xherdan Shaqiri and Pierre Hojbjerg.
Bayern were dismantled by a Lionel Messi-inspired Barca in the first leg of the UEFA Champions League semifinal, and play their home leg tonight needing to overturn a 3-0 deficit.
"I think mistakes were made. I did not understand why they sold Xherdan Shaqiri [to Inter Milan] and Pierre Hojbjerg [on loan to Augsburg]," Beckenbauer said over the weekend.
"They were great substitutes and they were missing [since January]. Of course you cannot expect to get as many injuries as we have. And some players were also out for a longer period of time than expected.
"Bayern were lucky that Thiago Alcantara and Javi Martinez returned. But we still did not have our full firepower, and you need it when you play in the Champions League semifinals against Barcelona.
"In these games you need full firepower and it is not enough to play with just the players that are available at the moment. They were punished [for recent transfers]. I am sure they will think about the future and the necessary steps to avoid this."
The World Cup-winning player and manager believes Bayern will have no choice but to sign better players during the next transfer window despite their Bundesliga dominance.
"I can imagine Bayern will buy new players," he added. "Some players are over 30, how long can they still play? The games are getting more and more intense. That's why you have got to have a big squad. But if half of the squad is injured than you have bad luck. So, I can imagine the squad will be changing."
Meanwhile, another Bayern and Germany legend Lothar Matthäus has suggested that Pep Guardiola is losing the dressing room.
"Guardiola has already said that their season is over and he's been rotating his side so much that he's unsettling the team," Matthäus told Sky Deutschland.
"His team no longer have the stability they had for over eight months, during which they delighted fans all over Germany. It looks like the players have lost a bit of faith in Guardiola, something he wasn't expecting."
