Executives from Bristol-Myers Squibb and Celgene were just a few minutes into a Q&A with investors about their newly announced $74 billion merger when longtime biotech analyst Tim Anderson raised a troubling question. One of Celgene’s crown jewels, the $8-billion-per-year multiple myeloma drug Revlimid, is facing patent challenges that could open it up to earlier-than-expected generic competition. So was Celgene really worth the more than 50% premium BMS placed on it?

The projected price-earnings ratio on Celgene in 2020 is just 6 times, Anderson pointed out, making it "the lowest multiple stock in the innovative biopharma space, which speaks to, I think, investor concerns about Revlimid."

Those worries are understandable, considering how big a role oncology played in BMS’s justification for the deal. During the conference call, CEO Giovanni Caforio, M.D., said it was not just Revlimid and Celgene’s other marketed products that made the company so attractive, but also the opportunity for six short-term product launches, including two CAR-T treatments for cancer: JCAR017 to treat relapsed diffuse large B-cell lymphoma (DLBCL) and bb2121 for multiple myeloma.

Perhaps, but Revlimid casts a long shadow that analysts have been unable to overlook. Celgene has been fighting challenges related to Revlimid’s dosing patents, and its fate could be sealed next month. If Celgene loses, Revlimid copies could enter the market in 2021—one year earlier than expected.

Meanwhile, Celgene’s efforts to expand Revlimid’s reach have not been entirely successful. In December, for example, Revlimid fell short in a non-Hodgkin lymphoma trial testing it alongside the Roche standby Rituxan.

Anderson, managing director of equity research for Wolfe Research, asked BMS’s Caforio on the call to provide long-term guidance on Revlimid, but the CEO declined to offer specifics. The company has done "a lot of due diligence on this. We feel good about what we’ve seen," Caforio said.

But after analysts had time to digest the deal, they weren’t feeling so great about the Revlimid risk. Credit Suisse analysts, for example, figure BMS could suffer a $3 billion revenue blow in 2021 if Revlimid generics come early and a $7.5 billion hit the following year.

Indeed, the sale itself may show how vulnerable Revlimid is. The odds of Celgene reaching a settlement that would keep generic competitors off the market in the short term must be lower than expected, Bernstein analyst Ronny Gal wrote Thursday; otherwise the company's top executives would not have been so eager to sell. If it loses the patent challenge, Gal said in an investor note, "then Bristol will need to deal with very concerned shareholders."

BMS estimated that the six late-stage products in the BMS-Celgene portfolio could bring in combined peak sales of $15 billion. But will that be enough to make up for a Revlimid sales decline? Leerink analyst Geoffrey Porges pointed out in a note that even if Revlimid were to hold onto its market exclusivity until the mid-2020s, BMS will still face considerable revenue pressure, because its blockbuster checkpoint inhibitor Opdivo and fast-growing blood thinner Eliquis will also be facing generic competition around that time. So the $15 billion from the new products is "unlikely to completely offset the revenue erosion from the losses," Porges wrote.

And there’s no guarantee all the late-stage assets will make it to blockbuster-land anyway. Even though Celgene made a splash in December with positive data from a trial of its multiple myeloma CAR-T bb2121, there are at least 10 potential competitors in development with a similar therapeutic target, making the competitive field particularly crowded. And Celgene’s CAR-T treatment for lymphoma, JCAR017—which it picked up in its $9 billion acquisition of Juno Therapeutics last year—will have to compete with Novartis’s Kymriah and Gilead’s Yescarta.

So it’s no surprise that by the end of the day on Thursday, few biopharma analysts shared BMS’s optimism about the Celgene merger. "The company insists that this acquisition is about the pipeline and synergies and that the next 3 years of Revlimid are key to paying for the deal in the short term only," Credit Suisse analysts wrote. But in light of the patent concerns and all the other challenges facing the oncology portfolio, they added, "we believe it will be hard for investors to view the deal through the same lens as management."