Shares of Chipotle Mexican Grill slipped more than 2 percent in premarket trading Monday after Oppenheimer downgraded the stock and said Wall Street is too optimistic about the company's earnings trajectory.

The company's shares have already been upgraded nearly a dozen times and their price has soared nearly 80 percent since Brian Niccol was announced as Chipotle's new CEO in February. But Oppenheimer analyst Brian Bittner doesn't think marketing efforts and new menu items will be enough to meet earnings expectations through 2020.

Currently, Chipotle's forecast is based on the projection that the company will have mid-single-digit same-store sales every quarter for the next two years. However, Bittner said that even with healthy sales, these estimates are too aggressive.

Bittner said the burrito chain's "high-flying" valuation was previously based on earnings per share of $20 through 2020, which he now sees as unlikely.

"The Street's margin expansion assumptions could be difficult to achieve without meaningful sales upside owing to labor headwinds, potential store-level investments and lack of menu-pricing upside," he wrote in a note to investors Monday.

Its stock was about flat in mid-morning trading at $452.46 a share.

Chipotle has continued to struggle, even with Niccol now at the helm. The burrito chain is once again trying to draw customers into its restaurants after another food poisoning scare in August. An outbreak of Clostridium perfringens, a foodborne disease that occurs when food is kept warm for too long, was traced back to a Chipotle in Ohio and stymied sales in the third quarter.

Almost 700 customers reported gastrointestinal problems, including nausea, diarrhea and fever, after eating at the Chipotle restaurant in late July, according to local health officials. The incident dealt another blow to Chipotle, which has spent years trying to persuade diners and investors it has improved its food safety practices.

Despite this setback, shares of the company remain up more than 57 percent since January.

Under Niccol, the restaurant hopes to woo customers back to the troubled burrito chain with new and limited-time menu items, as well as faster mobile and online ordering and a new marketing campaign. In September, Chipotle said it was bringing its spicy pork and chicken chorizo back to restaurants for a limited time.

"We respect management's marketing, digital and new product strategies, but don't view them as transformative enough," Bittner wrote.

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