Harrison has been cleared to return after recovering from coronavirus, but the attacker needs to be monitored over the next 24 hours before a decision on his involvement this weekend is made.
Ayling and Raphinha, meanwhile, are struggling with knee and groin issues respectively, with the duo both doubts for the clash with David Moyes's side.
"Diego Llorente, Robin Koch and Patrick Bamford won't be available. The rest of them I couldn't tell you if they will be or they won't be," Bielsa told reporters at his pre-match press conference.
"If I said they would definitely play or wouldn't play, neither of those things are certain. That's why I can't confirm either of the two alternatives."
Pascal Struijk, meanwhile, will again be absent for the Whites through suspension. body check tags ::