Humana, though, saw a far steeper drop than Aetna, falling 11% since midday Thursday, as compared to Aetna's 6% drop.
Not to mention, JPMorgan downgraded Humana from overweight to neutral.
"Of the 'deal stocks,' we have long held that only Humana shares have material downside if the DOJ were to block the pending acquisition by Aetna," JPMorgan analyst Gary Taylor wrote in a note.
Others say that since Aetna's purchase price of Humana comes at a premium to its trading price, it makes sense that Humana's share price was hit harder by the news that the deal could fall through.
"I definitely believe Humana is worth less on a standalone basis than Aetna's offer price," analyst Vishnu Lekraj of Morningstar said in an interview.
Now, as the deal seems less likely to go through, investors are selling off their stock in Humana.
"Back in May we still saw a better than 50/50 chance of deal approval," Taylor wrote in a note. "We now see a much lower probability."
He added that JPMorgan now sees a 10-15% chance that the deal will be approved by the Department of Justice as long as Humana makes divestitures.
Until the end of this week, investors seemed positive on the Aetna-Humana deal going through.
"Investor sentiment has until recently been more positive around the prospects for the Aetna-Humana merger to receive regulatory approval so the recent headlines are clearly requiring a rapid reassessment from investors on the prospects for the deal," analyst Scott Fidel of Credit Suisse wrote in a note.
Analyst Frank Morgan of RBC Capital Markets agreed.
"The conventional wisdom was that this deal had a higher chance succeeding than Anthem-Cigna," Morgan said in an interview.
According to Taylor, if the deal does indeed fail, Aetna will have a tough choice to make.
"If the Humana deal is blocked, Aetna must decide whether to litigate, and ultimately owes a [$1 billion] pre-tax ($2/share after-tax) termination fee to Humana if unsuccessful," Taylor wrote in a note.
He added that Aetna shares could go as low as $99 per share if that were to occur.
As for Humana?
"Humana and its Medicare Advantage business are still a very attractive business," Morgan said. However, he added, it isn't likely that the company will be purchased by another large managed care company.
Analyst Ana Gupte of Leerink Partners, however, does not think this is the end for the Humana deal. "In our view, Aetna and Humana continue to have productive dialog with the DOJ," Gupte wrote. "They are reported to have embarked on a process of asset sale to address specific antitrust concerns from the DOJ."
Aetna and Humana could not be reached for comment on the deal. The Department of Justice declined comment.