Investors flooded into Achillion Pharmaceuticals Inc. (NASDAQ:ACHN) last week to get a piece of the lone biotech with unpartnered clinical-stage HCV assets. However, at least one buysider thinks Achillion missed its window of opportunity.

Achillion jumped $1.37 (48%) to $4.25 last Monday on news that Merck & Co. Inc. (NYSE:MRK) was buying HCV company Idenix Pharmaceuticals Inc. (NASDAQ:IDIX) for about $3.9 billion in cash (see Cover Story).