"If I were a shareholder of Microsoft, I would be sending a note to the board of directors going, 'Please do not do this,' " McNamee said.
Salesforce, which has a market capitalization of $48.7 billion, is already too big. Microsoft should be building in house instead of acquiring a company such as Salesforce, said McNamee during CNBC's "Squawk Alley."
"They should be building things internally, and they have lots of opportunities to do that," said McNamee.
To Microsoft, Salesforce's culture might be more appealing than the products.
Yet, since Microsoft isn't looking for a new CEO, the company is certain not to benefit from the culture built by Marc Benioff.
"I think in fact a large number of people would leave right away and they'd have all the products and the products would be of some value, but I don't think you can justify 50 or more billion dollars to acquire a company at that stage," said McNamee.