Running backs coach Kyle Caskey made the announcement Friday.
Swift practiced Wednesday in full and then spoke with the media following practice. He was not at practice Thursday and later in the day was listed with a concussion. On Friday morning, before Swift had been ruled out, Lions coach Matt Patricia said it would be "difficult" for Swift to clear protocol in time to play against the Panthers.
Patricia and Caskey both declined to say how or when the concussion happened. "Just something that came up and wanted to make sure that we're doing our due diligence there, making sure that he was OK and that's just how they saw it," Patricia said. "We'll be careful and make sure he's out there as soon as he can based on the protocols and the protocols are what they are and the timeline, it's pretty tight."
Swift was coming off his first career start against Washington, rushing for 81 yards on 16 carries and catching five passes for 68 yards and a touchdown. The Lions had been giving him more work each week to get him ready to take over the starting role. It had been a process, in part, because of the lack of preseason games and spring workouts combined with time Swift missed during training camp due to an injury.
He has 70 carries for 331 yards and four rushing touchdowns this season along with 31 catches for 275 yards and two receiving touchdowns.
When the Lions told then-starter Adrian Peterson that Swift would be taking over the majority of the work, Peterson sensed it was coming. He saw Swift's talent from when he arrived in Detroit prior to Week 1.
"It made sense to me. Actually, when I was told, I was like, 'You guys should have did this two, three weeks ago, you know,'" Peterson said. "So I'm here to try to win, that's my main objective. Of course I want to play, I'm a competitor.
"So I heard the news [about the concussion], the first thing was, 'What's going on?' It was my concern for his health. It kind of threw me off. But after that, just, OK, the young guy's down. As a running back room, we have to step up and handle our job."
Peterson said he didn't know the details of what happened to Swift, but said he was told on Thursday he might be playing a lot more Sunday.
In Swift's place, the Lions will use a combination of Peterson and Kerryon Johnson, who was last year's starter.
Caskey said they are still determining how they'll be using their backs Sunday but said Peterson is "going to lead it off and do what he does," and that Johnson will come in and play as well.
Caskey also mentioned Jonathan Williams, who has been on the roster but not used as a rusher, could handle some of the workload.
"Hopefully we get a flow going one way or the other," Caskey said. "And see how it goes."
The Lions have also ruled out wide receiver Kenny Golladay (hip) and Danny Amendola (hip). Golladay had shown potential progress earlier in the week, practicing on a limited basis Wednesday. He also did not practice Thursday or Friday.
"We're trying to be smart with everybody here, knowing that we have two games really close together back-to-back," Patricia said.
The Lions have a quick turnaround to play the Houston Texans on Thanksgiving Day.
In better news for Detroit, receiver Marvin Jones (knee) and tight end T.J. Hockenson (toe) practiced Friday and are good to go, giving the Lions at least part of their core offensive group for sure. With Golladay and Amendola sidelined, expect to see more Marvin Hall, Quintez Cephus and Jamal Agnew at receiver. It's also possible the Lions could bring up veteran receiver Mohamed Sanu from the practice squad.
Detroit listed quarterback Matthew Stafford (right thumb) as questionable, but he's expected to start Sunday.