"There was a big inventory build in the third quarter, so it makes me think it's closer to 3 [percent] than to 4 [percent] in the fourth quarter," Hassett said on "Squawk Box." "I think you lose about a half a percent."
On Friday, the government's first estimate of third-quarter gross domestic product showed a faster-than-expected annual growth rate of 3.5 percent.
The data revealed that growth was powered in part by stronger-than-expected consumer spending, which helped cover for stagnant capital spending.
Asked whether that's a sign of at least 3 percent growth nearing an end, Hassett gave a definitive "no." But sustainable growth, he contended, must be supported by business investments because consumption is "less durable."
If Hassett's estimates hold, and there are no revisions, GDP growth for all of 2018 would be around 3 percent. The economy posted advances of 2.2 percent and 4.2 percent respectively in the first and second quarters.
As the Federal Reserve considers raising interest rates for a fourth time this year, investors have grown apprehensive about a possibly more aggressive tightening path next year.
In turn, the stock market has suffered wild swings. While starting the week trying to reverse Friday's decline, the still stands to suffer a rough October unless things really rebound by the end of the month on Wednesday.
Hassett, himself a former senior economist for the Fed, refused to offer advice about how the central bank should act. Yet, he did offer some general insight.
"When you have a lot higher economic growth, those natural rate of interest models tend to give you higher interest rates as well," he said. "But where it all ends up, that's up to the Fed."
The Fed under Chairman Jerome Powell has faced sharp criticism from Trump, who wants central bankers to stop hiking rates. Trump has argued the stronger growth in the economy is not causing problematic inflation.