Ezgjan Alioski appears set to make his first Premier League start in Leeds' home game against Manchester City on Saturday.
Harrison has played a starring role for Bielsa's side in their first three Premier League matches after City agreed in July that the 23-year-old could start his third season-long loan at Elland Road.
"When a player comes out of the XI and another player comes in and you have to give an opinion on this, we have to mix a lot of common places," Bielsa said.
"Harrison is very important, we're sad that he's not going to play. Alioski is able to do as well or better than Harrison, (but) then after this there is the game, where you have to show it."
Bielsa has no new injury concerns, but admitted the continued absence of playmaker Pablo Hernandez due to a groin strain was a blow.
"Pablo is a very important player for our offensive game," Bielsa said. "More than his experience, he brings a lot of creativity. Not having him means we will be less creative."
Leeds sit seventh in the table, taking six points from their first three matches while adopting the same high-press, high-energy tactics that swept them to the Sky Bet Championship title.
That same approach saw them lose narrowly at champions Liverpool on the opening day and Pep Guardiola's side can expect more of the same at Elland Road.
"It's one of the biggest games in the Premier League," Bielsa said. "We will try to play the way we always do. We wouldn't know how to do it any other way."
Guardiola's admiration for Bielsa has been well-documented since the Argentinian arrived in West Yorkshire just over two years ago.
Bielsa played down the notion he was Guardiola's mentor, insisting the younger Spaniard's success had been achieved on his own terms.
"First of all he's imaginative. He's able to instantly create solutions to problems that he comes across," Bielsa said.
"Another thing that distinguishes him as a top coach is that what he proposes, he is able to implement.
"Guardiola imagines football in a freedom type of way, but to imagine football this way doesn't mean the players are going to act the same way.
"To propose a solution to the game, to be in line with what the player can do and that they don't need a lot of time to incorporate it.
"For me, what I have just said is one of the biggest praises you can give a coach and I don't know too many coaches, in my humble opinion, who would be deserving of this type of praise."