The American Olympic downhill specialist injured his right knee last week during a training session in France and returned home for treatment.
"Marco will miss this season," said Melinda Roalstad, medical director for the U.S. ski team.
"I was training and it was a foggy day," Sullivan said. "I hit a small compression and crashed, but it wasn't really a crash because I went down, slid on my hip and my outside ski yanked away from me real quick, hyper-extending the leg.
"I bounced right back up and I knew I was hurt, but I didn't know how serious it was.
"I skied down a couple more times outside the course, free-skied the mountain, trying to feel it out and realized it was serious."
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