The Argentinian believes his side were outplayed by a superior opponent in his club's defeat at the Vicente Calderon, and is unsure of how to turn things around
The Basques have started the season in poor fashion, slipping to a 5-3 loss to Real Betis before their trouncing in the Spanish capital on Monday.
And the Argentine believes that Diego Simeone's side were far superior on the night in several respects, and admitted that he was unsure of how to fix his team's problems.
"At no time were we in this match," said the Argentinian manager, according to AS. "They were more powerful than us in most aspects of the game.
"For a team that is in the cycle we are, it is impossible not to be generate concerned. I have no expectations to resolve this through the arrival of signings."
Athletic Bilbao's next match is a trip to HJK Helsinki in the Europa League, with Bielsa's side holding a 6-0 lead from the first leg.