Forshaw and Junior Firpo both went off injured, clutching the back of their left legs, in the space of two first half minutes at the London Stadium.
Marcelo Bielsa was forced to replace the senior pair with teenagers making their Premier League debuts in Lewis Bate and Leo Hjelde, but the Whites still managed to pick up a vital 3-2 win in the capital.
Speaking after the game Bielsa said the problems were muscular and the early double substitution was difficult to take.
“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,” said the head coach.
Firpo posted on his Instagram account after the game, saying: “Also, sad about the injury, I will work very hard to come back stronger.”
Forshaw has now followed suit, with more encouraging news for Whites supporters.
“Thanks for all the messages, it’s only a small hamstring injury,” he said.
“I’m fighting to be ready for Saturday, if not I’ll be back straight after the break.”
The former Brentford and Middlesbrough man spent almost two years out of action through a complex hip injury before returning to Bielsa’s side earlier this season and once again becoming a mainstay.