Shares of Whole Foods Market rocketed 28 percent on Friday after Amazon said it plans to acquire the grocery store chain for $42 a share, in a deal valued at $13.7 billion.

Amazon's offer represents a 27 percent premium to Whole Foods' closing price on Thursday. With Whole Foods shares trading around Amazon's offer price, investors appear to be speculating that another suitor could make a play for the grocery chain.

Among those that could potentially make a bid are Wal-Mart, which has been focusing its deal making on bulking up its digital business, and JAB Holdings, a private equity player that has been shifting its focus to restaurants and food and beverage businesses.

Whole Foods has been under pressure from activist investor Jana Partners and money manager Neuberger Berman, which have called on Whole Foods to sell itself. The investors have criticized Whole Foods for its poor performance and have suggested the chain could be merged with another grocer.

Whole Foods attempted to placate Jana last month May by shaking up its board of directors. However the hedge fund balked, saying that newly appointed board members did not have the grocery store experience to spark a successful turnaround.

Jana Partners did not immediately respond to CNBC's request for comment.

"This partnership presents an opportunity to maximize value for Whole Foods Market's shareholders, while at the same time extending our mission and bringing the highest quality, experience, convenience and innovation to our customers," John Mackey, Whole Foods' CEO, said in a statement.

Mackey will remain CEO of the grocery store chain after the deal closes, and the store will continue to operate under the Whole Foods brand.

Amazon has long been pushing to expand its online grocery business, seeing it as an emerging opportunity. Currently, few people purchase their groceries online even as more shoppers switch to buying other goods that way. About 12 percent of U.S. grocery shoppers bought their groceries online at some point in 2016, according to Cowen and Company.

Millennials, in particular, were the biggest group of consumers to use online grocery shopping last year. Older millennials, over age 25, were twice as likely to buy online. This generation is starting to get married and have children and will be growing their grocery spend over the next decade.

"The brand is a good compliment to Amazon and would allow them to more aggressively target fresh food delivery to the at-home market," Darren Tristano, chief insights officer at Technomic, told CNBC.

Some analysts had seen Wal-Mart being best positioned to compete in the next phase of growth in online shopping because it was going to be able to use its vast footprint of stores to help distribute products ordered online. Also, Wal-Mart has been successful with its so-called click-and-collect model, where shoppers order online but stop by the store to pick up their orders.

"We feel great about our position, with more than 4,500 stores around the country and fast growing e-commerce and online grocery businesses," Walmart told CNBC.