The proposed $74 billion takeover of US biotech Celgene by compatriot BristolMyers Squibb (BMS), announced last month, may be looking somewhat shaky.

Analysts and investors up to now have not been singularly impressed by the planned transaction, touted as the largest healthcare buy on record, and it now seems that an activist investor could step up to torpedo it.

On Feb. 2, news agency Bloomberg revealed that hedge fund Starboard Value – which it called “a known agitator” – had taken a stake in BMS ahead of the deal’s announcement. Neither the size of the stake nor what alternative plans the fund may have for the drugmaker have been disclosed.

Among Starboard’s most recent moves was to convince Dublin-based Perrigo to divest its prescription drugs business. It also pressured California’s Depomed to put itself up for sale.

In the pharma marketplace, the plan to buy Celgene is generally regarded as a way for New Jersey-based BMS to gain more clout in its immuno-oncology portfolio and help it keep pace with US Merck & Co. Several analysts however, have expressed doubt that the biotech would get a healthy boost from being tied to the drugs major. The shares of both parties to the deal have underperformed recently.

Some market watchers have warned that the lower growth projections announced by BMS in late January, which pressured the company’s share price, could make it a potential takeover candidate once it swallowed Celgene, though others saw little appetite for mega deals in the marketplace at present.

Credit Suisse analyst Vamil Divan, for one, appeared to be more confident about the prospects, writing in a note to clients that he and his colleagues “continue to expect deal to close as planned,” despite the activist investor in the wings.