SHANGHAI, China — The slump in SoftBank Group Corp.'s shares could prompt Masayoshi Son to play an ace card — cashing in part of his stake in Alibaba Group Holding Ltd.

Son is likely to sell Alibaba stock to help pay for another buyback in an attempt to bolster SoftBank shares, according to Jefferies Group analyst Atul Goyal. It’s a surprise the Japanese technology giant’s shares are “languishing” despite its large stake in Alibaba, Goyal wrote in a note. The shares have become “decoupled,” and SoftBank is seeing little upside from its holding, he said. SoftBank’s stock is up 16 percent this year, while Alibaba’s has surged 45 percent. SoftBank’s market cap is about $82 billion, though its Alibaba shares alone are worth about $128 billion.

SoftBank’s February announcement of a record 600 billion yen ($5.5 billion) buyback sent its shares to a peak in April, but the stock has since lost most of the gains. Investors have been spooked by the one-two punch of Uber Technologies Inc.’s plunge after an initial public offering in June and WeWork’s meltdown that forced a bailout by SoftBank. The poor performance of Son’s two marquee investments called into question the billionaire founder’s deal-making approach just as he’s trying to raise a successor to his $100 billion Vision Fund.

As the current stock price is “well below” the average price paid in the stock repurchase earlier this year, “we will not be surprised if SoftBank Group funds yet another buyback, perhaps in February 2020, by selling some more stake in Alibaba,” Goyal said.

Both SoftBank’s sale of part of its stake in the Chinese e-commerce giant earlier this year and using Alibaba shares as collateral for a loan indicate Son’s willingness for such a move, Goyal said. In addition to buybacks, proceeds could be used for investment in the second Vision Fund, the analyst said.

Responding to criticism about his reluctance to exit successful investments, Son in June 2016 unveiled a plan to sell 73 million American Depositary Shares in the online mall operator. The complex transaction, structured so that he could retain some upside if the stock rose, took three years to complete. SoftBank booked 1.2 trillion yen in pre-tax profit from the deal and still holds about 26 percent of Alibaba.

By Kurt Schusslerand and Pavel Alpeyev; editors: Lianting Tu, Edwin Chan, Peter Elstrom and Vlad Savov.