Activist hedge fund Starboard Value has built a 9.8 percent stake in small health insurer and pharmacy-benefits manager Magellan Health MGLN . Though Starboard partners and founders Jeffrey Smith and Mark Mitchell said in a regulatory filing that they do not at the moment have an official plan for Magellan, a source familiar with the matter told CNBC that the fund could consider advocating for a potential sale or managerial changes to help boost shareholder value. Magellan shares, which were down 35 percent this year, jumped nearly 8 percent Thursday following news of the Starboard stake.