Bielsa, whose side were leading in Lisbon with less than 20 minutes to go, withdrew his midfielder for defender Mikel San Jose Dominguez, which led to late goals from Sporting duo Emiliano Insua and Diego Capel.
"The team suffered in the absence of Herrera," Bielsa told reporters. "I thought we would win more balls while [Sporting] were attacking.
"We have to win at home now, but that should not be viewed as an unachievable objective.
"If there is one conclusion that it is impossible to draw after this game tonight, it is that Athletic underestimated their opponents."
The sides will meet at the San Mames for the second leg in a week's time.