Leon Cooperman says future stock market returns will be 'unimpressive for a long time'

CNBC's full interview with billionaire investor Lee Cooperman

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Billionaire investor Leon Cooperman warned on Monday that long-term market returns could be lackluster because this year's incredible comeback will likely rob returns from the future.

"The overall market, we've been pulling a lot of demand forward. I would expect that future returns will be relatively unimpressive for a long time," Cooperman said Monday on CNBC's "Squawk Box."

Cooperman said when people realize most of the gains are because of government help, valuations for the stock market will come down.