The 25-year-old forward picked up the problem during the Hammers’ Europa League third qualifying round first-leg tie against Astra Giurgiu away from home on Thursday.
Valencia was taken out of the pitch on a stretcher, as the game ended in a 2-2 draw.
Bilic has revealed the extent of the injury to the Ecuador international and has admitted his frustrations.
“It looks serious but we won’t know for sure until he has a scan,” the Croatian told The Sun.
“He will have one in the morning and we can only hope it’s not very bad. I think Enner twisted his knee and I am worried.
“It’s the biggest reason why I’m not happy. I’m not happy with the result, of course, but that is the main thing because it doesn’t look good. I’m disappointed and it wasn’t a great night.”
Big blow for West Ham United
According to The Sun, Valencia may have damaged his cruciate ligament and could be out for months.
If that is the case, then it will be a huge blow to West Ham and their chances of a top-eight finish in the Premier League this season.
The 2014 summer signing was one of the Hammers’ best players during the 2014-15 campaign.
The former Pachuca star scored four goals and created 17 chances in 32 Premier League appearances for the London outfit last season.
With the new Premier League season so close, West Ham fans will be worried with the injury to Valencia.