"It would seem to us if this deal doesn't get approved, you're likely to see Sprint shares fall to something like $4 or so," said Craig Moffett, founding partner at MoffettNathanson.
Sprint stock was about 12 percent lower, under $6 per share, in premarket trade Monday after it agreed to be bought by T-Mobile U.S. in an all-stock deal worth $26.5 billion. T-Mobile's stock was 2 percent lower. The deal values the combined company at about $146 billion.
T-Mobile and Sprint — the nation's third- and fourth-largest wireless companies — have considered a combination for years, including a failed attempted merger in 2014 under the Obama administration.
In a "Squawk Box" interview, Moffett said he expects a 50/50 chance of regulatory approval, saying Sprint has a lot more riding on the merger than T-Mobile, which he sees as a much more attractive stock.
The combined company, which is touting an ability to create a large-scale 5G network and thousands of U.S. jobs, will take the T-Mobile name and will be run by T-Mobile CEO John Legere.
"T-Mobile has a pretty good path forward on a stand-alone basis and Sprint doesn't," Moffett said. "Sprint has generated positive free cash flow in years and years and they were burning cash before they even started spending money on their network."
Walter Piecyk, a research analyst at BTIG, told CNBC that he puts a less than 40 percent chance that the deal will be approved. "You're going to see that reflected in the stock prices today," he said.
"T-Mobile can live without Sprint. Can Sprint live without T-Mobile?" he added.
—The Associated Press contributed to this report.