Drafted in the second round out of Ohio State, McMillan never got the chance to line up on defense after suffering the injury on punt coverage in the opening minutes of the first quarter. He walked off the field under his own power and was taken to the locker room. While his status was initially listed as questionable, he did not return as the Dolphins beat the Falcons 23-20 on Thursday.
"Nobody's told me anything yet," said Dolphins coach Adam Gase, when asked whether he could confirm a report of a torn ACL. "So you guys seem to have better sources than I do. I'm the head coach, so I don't know.
"We were talking about how he was possibly gonna have to be on [special teams]. We knew we had to get him some reps at it, especially in the game. It's tough to see a young player go down, not knowing what happened."
If McMillan's injury is determined to be serious, Gase will be looking for someone to step up.
"I mean, that's the NFL," he said. "Next man up. That's how it goes. We'll see what happens over the next couple days."
McMillan had clearly impressed coaches in training camp, where he has been the starter since day one. Mike Hull, an undrafted linebacker heading into his third season with the Dolphins, is listed behind him on the depth chart but has just one career start, which came last season. Lawrence Timmons, who is currently starting at outside linebacker, is another option. He has the ability to move inside after playing on the interior for much of his tenure with the Pittsburgh Steelers.
"Hopefully it's something that's very temporary," defensive end Cameron Wake said of McMillan's injury. "Who knows what's going on. I don't know. Any time any player goes down, your heart goes out to them."
The Dolphins host the Baltimore Ravens next Thursday in Week 2 of preseason play.