Shares of Elanco Animal Health (ELAN) have been unusually volatility over the past two days, William Blair analyst John Kreger tells investors in a research note. On Monday, the FDA approved a generic version of the company's largest product, Rumensin, and yesterday, the media reported speculation that Bayer (BAYRY) has approached Elanco about combining their animal health businesses, the analyst explains. He believes that while a negative development, the Rumensin generic had been expected and thus does not change his modeling assumptions. The Bayer speculation is more surprising but does not seem likely to happen "for a number of reasons," says Kreger. Elanco's largest challenge is bolstering its organic revenue growth, and buying a large company that is growing at a slower pace would only add to this challenge, according the analyst. Further, Elanco has committed to deleverage its balance sheet in the coming year, he adds. And lastly, Kreger believes there could be antitrust challenges from such a large combination. He keeps a Market Perform rating on Elanco shares.