The memorandum goes into a detailed analysis of factors that can help Red Lobster's business improve, such as a decline in various input costs, along with a reduction of complimentary meals. "The Red Lobster management team believes that the identified cost savings are a conservative estimate of true potential with additional upside above identified opportunities," the memorandum says.
Those benefits lead to a much rosier profit outlook than Darden investors had been told to expect. Specifically, the memorandum estimates the company's earnings before interest, taxes, depreciation, and amortization (ebitda) can be $150 million after it addresses some low-hanging fruit. That compares with an "unadjusted" ebitda level of $125 million for fiscal 2014, which ended in May.
The memorandum said Red Lobster can be even more profitable, reaching a target ebitda of $200 million as the company realizes further cost savings.
Darden shareholders, meanwhile, were told that Red Lobster's profit was in decline with no sign of improvement. In Darden's Aug. 4 presentation, it said Red Lobster "was expected to have less than $100 million of ebitda as of the transaction close." Darden went on to say that "given deteriorating trends at Red Lobster and multiple turnaround attempts, the divestiture is projected to be medium and long-term accretive."
The difference in outlooks is critical because a higher level of ebitda would likely mean a greater contribution to Darden's profits if Red Lobster had remained part of the company. Similarly, investors might have hoped for Darden to sell Red Lobster for a higher price if its prospects had become rosy.
Read MoreDarden sells Red Lobster to Golden Gate
The memorandum also raises questions about Darden's disclosure. Listed companies commonly communicate with select groups of investors, but normally need to make disclosures "promptly" if any nonpublic information is disclosed, according to a Securities and Exchange Commission rule known as Regulation FD. The rule applies if selectively-disclosed information is material to a company's financial condition. A spokesman for the SEC declined to comment to CNBC Digital about Darden.
The memorandum contains a letter dated June 25 from Red Lobster to Deutsche Bank, the deal underwriter, stating that the information it provided is "either (i) publicly available or (ii) not material (although it may be sensitive and proprietary)." Deutsche Bank declined to comment.
Darden wasn't required to seek shareholder approval for the Red Lobster sale. Starboard, which owns 8.8 percent of Darden shares, attempted to put the matter to shareholder vote and told the company it had secured enough shareholder support to call a meeting. But Darden announced its deal to sell Red Lobster before any such meeting took place.
The activists have criticized the Red Lobster sale since the deal. In May, Barington said the deal "severely undervalues Red Lobster" and "does not retain future upside for shareholders." Starboard said on July 15 that the company "decided unilaterally to sell Red Lobster at a fire sale price."
Some Wall Street analysts have also criticized Darden for its move. "Management's decision to ignore shareholder concerns and go forth with an undervalued sale of Red Lobster as opposed to waiting for operations to improve or entertain monetization without fully disposing the brand during a depressed earnings period will likely result in meaningful changes at the board level and among senior management," Buckingham Research analyst Matthew DiFrisco wrote in a May investor note.
Darden announced that CEO Clarence Otis would step down from the company on July 28, the day the Red Lobster deal closed. Even so, Darden shares have remained weak, falling 13 percent since the start of 2014 while the S&P 500 has gained 7 percent.
The activists, meanwhile, have pushed for further change. Starboard has nominated 12 directors, enough to replace Darden's entire board, for a vote at the annual shareholder meeting on Sept. 30. Barington has said it supports all of Starboard's nominees.
—By CNBC.com's John Jannarone