"If they hadn't announced the spin it would have been a fiasco and Mayer probably would have been fired," said one large Yahoo shareholder. "Now, Marissa and (CFO) Ken Goldman are on the clock for the next three quarters."
The Alibaba spinoff was critical because it effectively created about $14 per share in value through cost savings (assuming the stake would have been taxed at 35 percent in a traditional sale).
That is the most valuable of a number of steps Starboard suggested in the fall. The activist also urged the company to pursue a tax-free spinoff of its stake in Yahoo Japan and consider a merger with rival AOL. Assuming a normal tax rate, there is about $2 per share to unlock from a tax-free divestment of the Yahoo Japan stake and a merger with AOL could generate several dollars per share in synergies, according to analyst estimates.
It's unclear whether Starboard will be satisfied with the Alibaba spin or continue to pressure the company. Starboard declined to comment.
Read MoreYahoo to spin off its remaining Alibaba stake
Other shareholders have mixed views on what Mayer should do in coming months. Some hope for a tax-free spinoff of Yahoo Japan or a potential merger with a suitor like AOL.
But even if neither of those hopes materialize, it's likely that Yahoo's core business will come into much greater focus. With the Alibaba stake in the picture, core Yahoo had been almost too small to matter.
Now, it's much easier to notice what's going on in the core business. After subtracting the roughly $40 per share in value that the Alibaba stake is currently worth, there's about $11 per share remaining in the stock. The company also has about $6 per share in net cash and the Yahoo Japan stake is worth about $7 per share. That suggests the market continues to put a negative valuation on core Yahoo.
What can be done to improve the valuation on the core business? The main issue is that the company continues to incur higher costs while advertising revenues stagnate. In fairness, there are some promising signs: the company said Tuesday it expects revenue from new areas like mobile, video, and social to offset traditional display revenue declines in 2015.
The trouble is that the company still struggles to generate enough revenue for the ads it sells. In the fourth quarter, the price per display ad from Yahoo properties fell 20 percent. That may be a result of difficulty competing with the likes of Google and Facebook, which are adept at selling ads based on a fairly specific profile of the person who sees them.
After giving investors the gift of Alibaba just as they wanted it, the pressure on Yahoo is likely to ease up considerably. But with the core business now in focus, Mayer will face a new test altogether.