Morgan Stanley Matthew Harrison expects shares of Biogen (BIIB) to end the trading day flat or slightly higher. The stock was down $26.61, or 8%, to $291.50 in pre-market trading.

With Biogen's aducanumab more potent than Eli Lilly's (LLY) solanezumab, the Alzheimer's drug still has potential for success, Harrison tells investors in a research note. He points out that Lilly management noted that "the study results, including many secondary clinical endpoints, directionally favored solanezumab." Harrison sees hope in the amyloid hypothesis.

The analyst has an Overweight rating on Biogen.