Stewart injured the leg on Sunday in the EuroLeague Women championship game in Hungary. She flew to Los Angeles where test results confirmed the injury.
The Seattle Storm announced the diagnosis on Wednesday.
"As we all know there are highs and lows throughout a career," Stewart posted on her Twitter account. "This is just another obstacle that I will overcome. I'm thankful that I have so many people in my corner to help me every step of the way. I'm feeling every emotion possible at this point but just know that the bounce back will be real and I'll be back better than ever."
She will undergo surgery performed by Dr. Neal ElAttrache at a date to be determined. She is expected to make a full recovery for the start of the 2020 season.
Stewart was playing for Dynamo Kursk, a Russian club, when she appeared to land awkwardly on the foot of fellow WNBA star Brittney Griner after attempting a jumper late in the first half against UMMC Ekaterinburg. She immediately grabbed at her lower right leg while grimacing in obvious pain.
Stewart was attended to by trainers on the court before being helped to the locker room while not putting any weight on her right leg.
"This is a tough time for Stewie and our hearts go out to her," Storm CEO and General Manager Alisha Valavanis said. "The Storm family is behind her and we will support Stewie in every way we can as she begins her journey back to the court."
The 24-year-old Stewart won four national titles at UConn, and was the No. 1 overall pick in the 2016 WNBA draft. She won both the league and finals MVP awards while leading the Storm to the franchise's third WNBA title this past season.
Stewart has played overseas in the offseason since she joined the WNBA. Her Dynamo Kursk squad fell 91-67 after she left the game.
The WNBA season begins May 24.
Follow Doug Feinberg on Twitter at http://www.twitter.com/dougfeinberg
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