Boeing's stock posted its biggest two-day drop in a decade, and several Wall Street firms are warning that investors are still not anticipating the risk now facing the company's core airplane model.

Even if Boeing's 737 Max planes don't remain grounded for long, Goldman Sachs warned on Wednesday that the two deadly crashes since October threaten a major part of Boeing's revenue in the years ahead.

"The 737 MAX is the largest product revenue and EBIT (earnings before interest and taxes) contributor at Boeing, causing substantial investor questions on the situation," Goldman Sachs analyst Noah Poponak said in a note to investors. "We believe investors were pricing in very limited risk to the 737 ramp-up profile, which now has greater risk within the wide range of possible outcomes following these incidents."

Key to Poponak's warning is his firm's estimate for how much the 737 Max fits into Boeing's future. Over 60 percent of the 385 Max aircraft in service around the world are now grounded but, more importantly to Boeing is the immense order book. Boeing has a backlog of more than 4,600 orders for the 737 Max, and Goldman Sachs believes the aircraft makes up 33 percent of Boeing's total revenue for the next five years.

"On the one hand, should the 737 MAX problem be contained to a software fix, this would likely be a quicker and less costly fix implementation," Poponak said. "On the other hand, if it is determined to be a more comprehensive design flaw that requires more redesign it could take long and be costly."

Goldman Sachs has a buy rating on Boeing with a $425 per share price target. It was trading at $380 per share in Wedensday's premarket, up 1.22 percent.

Here is what all the major Wall Street firms are saying about the Boeing 737 Max: