Shares of Celgene (CELG) - Get Report are tumbling 5.5% Thursday, thanks in part to some analyst coverage by Morgan Stanley. Analysts downgraded the stock to sell and maintained a $120 price target. Before Celgene's fall, that price target represented about 18% downside.

Additionally, the analyst initiated Biogen (BIIB) - Get Report with an outperform rating and $375 price target, implying about 15% upside from current levels. Shares are up 3.8%.

"This is really important," TheStreet's Jim Cramer said of the Celgene downgrade. The analyst missed the entire rally up in Celgene, from the teens to more than $140. Even though it would be hard for the analyst to put a buy rating on Celgene now, "Celgene is not a sell," Cramer reasoned.

The analyst makes a case for Revlimid's intellectual property. In plain english, he's worried about generic drugs pressuring Revlimid's sales, which represented more than 62% of total sales last quarter. The analyst makes the case that these pressures could come to light in 2020, while most analysts are modeling for full generic for 2026.

It's reminiscent of Teva Pharmaceutical (TEVA) - Get Report and Mylan (MYL) - Get Report . Shares of Mylan jumped more than 15% on news that the FDA approved its generic version of Teva's top-selling multiple sclerosis drug, Copaxone. Teva, not surprisingly, is down 15% over the past two trading sessions.

"This is the kind of downgrade I see," Cramer, who also manages the Action Alerts PLUS charitable trust portfolio, said of Celgene. They don't hold enough water. These concerns are rather far off, although the analyst does admit that this may be the case. In any regard, Cramer does not view Celgene as a sell.

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At the time of publication, Cramer's Action Alerts PLUS had no position in any companies mentioned.