Morgan Stanley analyst David Risinger attributes the 15% pullback yesterday in Mallinckrodt (MNK) shares to a "confluence of factors," including a reaction to Valeant's (VRX) plunge after its guidance cut, comments from short seller Andrew Left of Citron Research and new CDC guidance on opioid use. Responding to the short seller's focus on Acthar, Risinger notes that Mallinckrodt has already offered greater rebates and discounts on both Acthar and Ofirmev. The analyst also said he believes that Mallinckrodt's opioid downside should be limited due to its marginal exposure to the targeted extended release and long-acting opioids. Risinger reiterates an Overweight rating and $88 price target on Mallinckrodt shares.