On CNBC's "Fast Money," Nathan explained.
"When you think about the names that have done a lot of the heavy lifting, there is not a whole heck of a lot of revenue growth," he said, citing second-quarter results. "There's been a lot of earnings growth, getting close to double digits, but a lot of that's manufactured. And I don't like it. I think you're going to see 1,900 very soon before year's end."
As for talk that the Alibaba IPO—history's largest ever—might have marked the market's top, Nathan said, "It could've been the sentiment top."
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Nathan said that he wasn't factoring in technical indicators such as the Russell 2000's recent "death cross," or when its 50-day moving average dips below its 200-day moving average.
With Alibaba stock up 38 percent the day of its initial public offering and with Apple stock performing well after last week's unveiling of the iPhone 6, iPhone 6 Plus and Apple Watch, Nathan said that the names might be running out of steam.
"To me, these are two massive names," he said. "There's almost a trillion dollars in market cap wrapped up in those two names, and a lot of investors are very invested in them."
Brian Kelly of Brian Kelly Capital brushed off talk of an Alibaba top for stocks.
"If Alibaba came with all this fanfare and they didn't make money … then I would say, 'Yeah, potentially this marked the top.' There certainly is a hibernating bear inside of me that would love that symmetry to line up. I just think it's way too early to make that call," he said.
Small caps, however, were another story.
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"I think we should be worried, right now, about the Russell," he said. "That's telling you something."
Private Advisor Group's Guy Adami focused on bank shares and small-cap stocks.
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"The good news for the bulls is the financials," he said. "The Russell, to me, is the canary in the coalmine."
Adami said that he expected the Russell to cross below 108.