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Why one of Marissa Mayer's big defenders changed his mind

Ironfire Capital Founder Eric Jackson was once one of Yahoo CEO Marissa Mayer's biggest defenders. Now he's changed his tune.

That's because while Mayer improved morale during her first six months, her actual accomplishments during her two-year tenure leave much to be desired, Jackson told CNBC's "Closing Bell."

The biggest problem, he said, is sales.

"Sales have been on sort of a terminal decline and they've gotten worse under her watch," he said, noting that appointing Henrique DeCastro of COO and head of sales was a "disastrous move."

Read MoreYahoo's Mayer says sleep blunder 'unfortunate'

Yahoo's chief executive, Marissa Mayer, speaks at the 2014 Cannes Lions in June in Cannes, France.
Didier Baverel | Getty Images
Yahoo's chief executive, Marissa Mayer, speaks at the 2014 Cannes Lions in June in Cannes, France.

Jackson said Mayer has also spent $2 billion in acquisitions that have "really destroyed value in the eyes of shareholders." Wealth was transferred to venture capitalists, not shareholders, he said.

Now, shareholders are concerned that once Chinese e-commerce site Alibaba goes public and Yahoo makes money off its stake in the company, Mayer will waste the money.

"Why should Yahoo shareholders believe that after wasting $2 billion in acquisitions that suddenly the next $2 billion or $3 billion she is going to spend post-Alibaba are going to be any better?"

Read More Alibaba IPO could send Yahoo soaring: Gene Munster

Since one of the biggest discounts embedded in Yahoo's stock is the assumption that the tech company will have to pay a huge tax bill after the Alibaba sale, Jackson thinks shareholders should lobby for Alibaba or Softbank to buy Yahoo.

"There would be a tremendous tax savings if one of those two companies bought Yahoo and returned a lot of the cash back to shareholders," he said.

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Jackson first wrote about his Mayer problem in a column for Forbes titled "How Do You Solve a Problem Like Marissa?"

—By CNBC's Michelle Fox

Disclosure: CNBC has a content-sharing partnership with Yahoo's finance site.

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