The law of averages says that if you perform any extreme sport long enough, you're going to accrue injuries. Depending upon their severity though, that doesn't mean you'll get knocked down for good. Nyquist for one is still going strong despite the complete absence of one ACL.
He says that the key to working through injuries is no big secret: Stay fit overall, but focus on body-weight exercises that strengthen the various muscles and tendons that surround an injury so that it is stabilized. He cites Rob Darden, who tore both ACLs, as someone who has helped him intelligently deal with his own knee tear. "At one point in his life, Rob was a little overweight. But he really got focused. He showed me that I could still do what I do, even with my injury," Nyquist says. "I've been riding without my ACL for a while now, but I could only do that by supporting the muscles around the area."