Dolphins head coach Adam Gase didn't know the extent of the injury, which occurred on special teams, but following the game he discussed the mentality if it turned into a long-term absence, per Omar Kelly of the Sun Sentinel.
"That's the NFL; next man up," he said. "That's how it goes and we'll see what happens over the next couple of days."
Miami selected McMillan in the second round after a standout career at Ohio State. The 20-year-old Georgia native racked up 275 total tackles, 10 passes defended, six sacks and an interception he returned for a touchdown across three years with the Buckeyes.
He was expected to step right into a key role for the Dolphins defense.
Now the coaching staff must decide whether to let Mike Hull and Chase Allen battle to fill the void in the middle or shift Kiko Alonso or Lawrence Timmons to the inside, opening a spot for either Neville Hewitt or Trevor Reilly in the starting lineup.
Neither of those alternatives are as attractive as having McMillan start from Day 1. Instead, he'll begin the process of trying to get ready to make his debut next year.