Analysts are turning on Costco (COST 1.43%), concerned that the longtime market darling is finally vulnerable following's (AMZN 0.22%) news-rattling announcement, late last week, that it would be acquiring Whole Foods Market (WFM) in a $13.7 billion all-cash deal.

Costco stock was downgraded by Paul Trussell at Deutsche Bank on Monday morning, following a similar move on Friday by Matthew Fassler at Goldman Sachs. Major Wall Street pros downgrading a stock on back-to-back trading days is never pretty, and there are more boo birds chiming in. Analysts at RBC Capital and Cowen, while not effectively downgrading the warehouse-club giant, did single out Costco as a potential casualty in Amazon's big push into physical retail of groceries.

Exterior of a Costco warehouse club

Image source: Costco.

Everything counts in large amounts

Goldman Sachs' Fassler lowered his Costco rating from "conviction buy" to neutral on Friday, slashing his price target from $197 to $176. He feels that Amazon's push into consumables, Costco's own fading fundamental catalysts, and real-world rival Wal-Mart's (WMT 0.60%) push into online initiatives will cap Costco's near-term upside. Fassler doesn't see Costco disrupted overnight, but over time, Amazon's notoriously razor-thin markups and seasoned delivery experience should gradually eat away at Costco's warehouse-club model.

Monday was Trussell's turn to sour on the stock, as the Deutsche Bank analyst lowered his rating from buy to hold. He feels Amazon's move is a game-changer, gunning for Costco's stronghold in groceries. He also singled out Wal-Mart's recent in-store and online moves to gain market share at Costco's expense. Trussell sees Costco's online operations as lagging its brick-and-mortar peers. Absent the recent catalysts for investor excitement over Costco that included last month's special dividend, the recent shift in credit card partners, and the annual membership-fee hike, Trussell feels that his firm's clients are better off investing elsewhere. He is lowering his price target from $187 to $172.

RBC Capital's Scot Ciccarelli feels that Costco and Wal-Mart both stand to take a hit following Amazon's move. Between the high-end brand that Whole Foods brings to the table and Amazon's mastery of logistics, it may not be long before they both feel the pinch. He didn't downgrade Costco or Wal-Mart. He feels that they both have strengths to fight here, but life clearly isn't getting any easier for either company as Amazon's insatiable appetite grows.

John Blackledge at Cowen made cautious comments about Amazon's ability to boost its Prime subscriber base, something that will come at the expense of warehouse-club subscriber trends at Costco and at Wal-Mart's Sam's Club.

It hasn't all been pessimism for Costco and its smaller warehouse-club rivals. Baird analyst Peter Benedict feels that Wal-Mart and Costco are positioned well to compete for a supermarket niche that has become an online and offline battle.

Costco shares are still trading higher year to date despite the bearish analyst observations. Outside of a 1% decline in 2016, you have to go to 2008 to find the last time that Costco shares declined for an entire year. The problem for Costco investors is that it will take a long time before we see whether Amazon's new toy is a serious threat, and between now and then, uncertainty will keep stock gains in check.