The two promotion rivals meet at Elland Road tonight in a match that could see the Baggies make history.
Never before have Albion started a campaign by avoiding defeat in their opening 10 league matches.
But Bielsa, whose side lost at Charlton on Saturday, isn’t surprised Slaven Bilic’s team are doing so well.
And he has warned his players they are set to face their biggest test of the season so far.
“West Brom have six players who have impact when they have possession, the two full-backs, the two wingers, the centre-forward and playmaker,” Bielsa said.
“As well as these six players, they have two defensive midfielders who manage the ball well.
“Out of all of the matches we have played so far, this opponent will be the most difficult opponent to defend against.
“We are always forced to win after one defeat more, we are playing against the top of the table at home and we always find reasons.”
Leeds are famed for their fluid playing style under Bielsa.
But they tend to line-up in a 4-1-4-1 formation.
Integral to their passing game is midfielder Kalvin Phillips who sits in front of the back four and dictates play.
But they have been dealt a blow ahead of tonight’s game with Pablo Hernandez ruled out through injury.
The winger has been crowned Player of the Year and Players’ Player of the Year in each of the last two seasons.
His injury means Wolves loanee Helder Costa could start.
Up front, Patrick Bamford is likely to continue as the lone striker.
But he is under pressure from both Eddie Nketiah and ex-Albion forward Tyler Roberts.