Shares of Chipotle Mexican Grill (NYSE: CMG) are under heavy pressure Tuesday after hedge fund honcho David Einhorn presented it as his new short idea at the Valuing Investing Congress in New York.

Einhorn said the valuation of the fast-casual, Mexican food restaurant is at "nosebleed" levels and history has shown that restaurants that generate the most hype eventually come back to earth.

The biggest challenge facing Chipotle, according to Einhorn, is Yum! Brands' (NYSE: YUM) Taco Bell. He said Taco Bell is on a reassurance and recently posted same store sales growth of 13 percent, versus 8 percent for Chipotle.

Einhorn is also cautious on Chipotle's undocumented worker problem and insiders at the company have been selling.

While Einhorn is positive on Taco Bell's resurgance, he is not keen on parent Yum! Brands due in part to the impact the slowdown in China is having on its KFC brand.

Shares of Chipotle are down 5.6 percent.

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