Sanford, who will need surgery, was injured on the first day of training camp.
"That one's tough, I feel really bad for him," Blues Head Coach Mike Yeo said of Sanford. "He worked hard this summer and you could see physically, he looked more ready and more prepared for the NHL game. He looked bigger, stronger. He had a tough break. I was really excited about his potential for us this year, and now he's got some work ahead of him, but his season's not over. If he can put in the work and get through the surgery and rehab properly, then you never know... it's certainly a big blow."