The Argentine, who recently penned a new contract, took the site manager to task over the conditions his players were forced to contend with on the first day of pre-season
The Argentine was enraged to discover that scheduled work at the Lezama facility had not been completed in time for the start of pre-season and became embroiled in a heated dispute with the site manager that quickly became physical.
Bielsa called a press conference on Thursday to explain his actions and claimed that Athletic and the construction company convinced the site manager not to press charges against him because they did not want to draw any attention to the allegedly shoddy workmanship.
"The condition of Lezama is an insult to the players and I am responsible for them," Bielsa explained. "I was indignant because that was not recognised and began to say offensive things. I don't respect him because he did a bad job.
"I behaved badly and I admit that. I offended the person but he also said some bad things.
"He made no police report and that, I believe, is because of pressure from his company or Athletic, who wanted to avoid [any publicity over] what I'm reporting here."
Bielsa argued that the onus was on him to take the site manager to task over the delay as it is his duty to fight for his players, given poor facilities reflect badly on him.
"Doing our pre-season in these conditions at Lezama discredits me as a coach," the 56-year-old fumed.
"The work had deadlines. I had no vacation and I met with them a week ago and also spoke with those responsible for about four hours a day while in Argentina.
"One of the deadlines was on July 2 and I was told that only half of the work would be completed. But it is not that the work wasn't completed; it's that it was done badly, in the knowledge that it was being done wrong.
"When I saw that, I got angry because I have a squad here that is worth €300 million that has to work in these conditions."
Bielsa arrived at San Mames last summer and, after leading Athletic to the final of both the Europa League and the Copa del Rey, he signed a one-year contract extension.