Both sides had chances to score before goalkeeper Dean Henderson's costly mistake resulted in Hernandez netting the winner in the 82nd minute.
Manchester United loanee Henderson passed straight to Jack Clarke and the substitute showed excellent composure to find an unmarked Hernandez in front of an open goal.
And Bielsa believes changing the positions of playmaker Hernandez and striker Kemar Roofe made the difference as Leeds went top of the Championship.
Bielsa said: "There was a moment, very important for me. My colleague Pablo Quiroga told me to switch the position between Kemar Roofe and Pablo Hernandez and from that moment on, the play of the team improved.
"But if the final result had been a draw, we couldn't say that it would be an unfair result.
"We had to defend a lot today. When we started playing out, we just played long balls from our goalkeeper and it's not a good thing to evaluate our style.
"Because we played like that, we share the possession with the opponents. We had more chances to score than the opponents.
"The opponents dominated at the beginning of the game and at the end of the game, but between these two moments, without being excellent, we did an interesting performance.
"It is not convenient to make comparisons with other teams at this moment in the season. What I can say is that we played against a serious opponent.
"It was hard for us to find a solution to the project of play from Sheffield United.
"I'm always happy when I win and if this result puts us at the top of the table, even better."
Leeds goalkeeper Bailey Peacock-Farrell kept out David McGoldrick twice while Billy Sharp missed a glorious chance to head the opener in the first half.
The Blades almost secured a point at the death as substitute Conor Washington's overhead kick cannoned back off the crossbar.
Sheffield United manager Chris Wilder said: "I thought it was a really good game. Credit to them in their approach. From the start of the season, it's been positive.
"There was no way they were going to come and sit back. I thought it was a similar approach from our point of view as well and maybe decided by a bit of magic, taking a chance or a mistake and unfortunately for us it's the final one.
"We've made a big mistake and we have to accept that. Dean (Henderson) is a big boy and he'll get on with it.
"I'm not going to rub his head for him. It's part and parcel of being a professional footballer and especially a goalkeeper.
"Two teams who had periods of the game. I think we had bigger periods in the game than them and in those you've got to find a goal.
"I thought it was a proper game of football, with tackles going in, although I thought the referee had a really poor game. Some of the decisions were head-scratching at times.
"I think we deserved something from the game but we haven't got it and that's the harsh realities of this fixture."