Firpo joined Leeds for €15m from Barcelona last summer but the defender's first half of the season was disrupted by injury setbacks and coronavirus.
The 25-year-old had since started to build up a consistent run of appearances, Firpo starting for the seventh time in United's last nine games in Sunday's Premier League clash against the Irons at the London Stadium.
Firpo, though, saw his afternoon cut short with just a quarter of the contest gone, the left back pulling up and holding the back of his leg before eventually being substituted for Leo Hjelde in the 23rd minute.
His departure came as part of a double injury blow for Leeds whose Adam Forshaw was also forced off at the same time and replaced by Lewis Bate.
Despite losing both players against the division's fourth-placed side, an already injury-ravaged Leeds outfit recorded a thrilling 3-2 victory in East London as Jack Harrison netted his first senior hat-trick.
Firpo took to social media after the game to say: "Well deserved win today, amazing performance from the team.
"Very happy for you Jack Harrison, no one else deserves this more than you.
"Also, sad about the injury, I will work very hard to come back stronger."
Bielsa was asked about the injuries to Firpo and Forshaw at his post-match press conference and said: "Both of them came off with muscular problems.
"Of course, the game being so early on where we had constructed our best moments; to have to generate adaptation with two substitutions always has its cost."