Iraizoz was singled out by supporters after shipping two goals in only a few minutes at San Mames.
“Clearly, on some occasions, the crowd demonstrated little tolerance for our goalkeeper," said Bielsa, whose side have yet to escape from the relegation fight with only six points separating them from Celta Vigo in 18th spot.
"Some important saves in a long campaign should quieten those complaints.
“A result like this draws focus to our goalkeeper in an exaggerated way and perhaps unfairly, but that is that case.
“When you lose by 0-4 in an accessible game against an opponent who is at the same level, then it is difficult to express justifications.
“For this game, we created three times as many chances to score as Espanyol, and we did not convert any, whilst our rival got four goals."