As Jakeem Grant was tackled by Aaron Colvin on the return, Trey Franks rolled over Millard's left leg.
After being tended to on the sidelines for a few moments, Millard was helped to the locker room.
Sooners center Gabe Ikard, Millard's roommate and one of his closest friends on the team, was devastated.
“My heart's broken for him,” Ikard said. “He's one of those guys that loves the game and for it to happen on something like kickoff — you know, getting rolled up on a kickoff — that's tough to swallow for everyone on this team.
“He's the heart and soul of this team and he's definitely one of the leaders. We're gonna miss him a lot. We're gonna have to make some serious adjustments on the offensive side of the football without (No.) 33 out there.”
In the immediate aftermath, Aaron Ripkowski and Brannon Greene filled Millard's role in the offense.
In the short term, it worked as the Sooners ate up nearly six minutes of clock on their next drive after the injury to all but ice the win.
Moving forward without Millard won't be easy, though, especially with undefeated Baylor looming next.
Stoops said Millard will have surgery in the next few weeks to repair the torn ACL and another torn ligament in his knee and he expects Millard to still have an impact in the NFL.
Millard finishes his career with 538 rushing yards on 98 carries and 70 catches for 677 yards.
“It's tough,” Stoops said. “Trey's just so versatile. We don't have anyone else that's really like that, nor does just about anybody else, so that's pretty unique.”