That's not really accurate. We've seen plenty on instances of similar injuries and it not matter too much. Maybe he falls 10-15 spots. But he's still a first rounder.
Jeffrey Simmons tore his ACL while preparing for the draft. He was still a first round pick, even with teams knowing he would likely miss most of his rookie year and even if he did return for part of the season, he likely wouldn't even be super productive.
Jameson Williams was drafted 12th overall despite tearing his ACL in the NCG.
And unlike those cases, McCoy has the advantage of actually being fully healthy going into the Combine, where he can showcase his talents and skillsets. And will surely be ready to start mini-camp next fall.