The only way for Walmart (WMT) to slow Amazon's (AMZN) amazing momentum is to go wild and make one mega deal, says one of the tech giant's former employees.

"A player like Wal-Mart Stores Inc.'s (WMT) Jet.com has the potential to affect Amazon, but no one has the potential to dethrone Amazon," said Brittain Ladd in an interview with TheStreet. Ladd left Amazon in May for family reasons after working there for two years, first as the senior manager of strategy and expansion for AmazonFresh and then as the senior manager of Amazon Global Logistics for the Special Projects unit.

If Walmart had been bolder and scooped up Whole Foods Market Inc. (WFM) before Amazon, then it would be a scarier competitor for Amazon, Ladd said. But as things stand now, Walmart's best option is to acquire Costco Wholesale Corporation (COST) because right now Costco isn't connecting with the wealthier shoppers that populate Target Corp. (TGT) , Amazon's website, and Costco. Furthermore, Walmart needs to shock the market with an acquisition like Costco in order to make its company more exciting to shop from than Amazon. Right now when people think of online shopping, they automatically think Amazon, not Walmart.

"The main difference between Walmart and Amazon is that Walmart sells groceries, while Amazon changes consumer behavior," Ladd explained. When Apple Inc. (AAPL) released the iPhone there were a number of other phones on the market, but Apple made their product exciting enough that it was able to change consumer behavior and become the dominant phone seller. Walmart has to carry out the same task in the retail industry, and the best way to do that is through buying Costco.

If these other traditional retailers can't pluck up the courage to make bolder M&A moves to become more competitive, then in 10 years Amazon will absolutely be the largest company on earth and dominate retail and many other areas, including banking and pharmaceuticals, Ladd said. "Amazon is just a toddler at this point," he said. "That's how much room it has to grow."

Walmart's shares fell 0.2% to $76.20 by Tuesday's close.

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