Lacy, the Packers' leading rusher, went on injured reserve on Oct. 20 and subsequently underwent surgery on his left ankle. There was hope at the time that Lacy could return late in the season.
Shields, the Packers' top cornerback, hasn't played since he sustained a concussion in Week 1. He went on injured reserve two days before Lacy.
"It's unfortunate for Eddie and it's unfortunate for Sam, but based on all the medical information and the conversations to this point, that's where we are," McCarthy said.
McCarthy wouldn't say whether Lacy and Shields had setbacks or whether this decision was based on the need for another cornerback for Monday night's game at Philadelphia. The Packers lost cornerback Demetri Goodson to a knee injury in Sunday's loss at Washington. Dorleant has been on injured reserve since Sept. 5 because a hamstring injury he sustained in the preseason.
"Once again, these are more medical decisions," McCarthy said. "And this is really a decision that's been made from the input of our medical staff."
There's no guarantee that Lacy or Shields ever plays for the Packers again. Lacy will be a free agent this offseason, and Shields' concussion history may force the Packers to move on. Shields has one year remaining on the four-year, $39 million contract he signed in 2014, but his latest concussion was the fourth of his NFL career and second within nine months.
McCarthy said he did not know whether Shields would be cleared to play again.
"Really, the conversation with Sam, I know particularly mine with him, is he needs to get healthy for himself and his family," McCarthy said. "That needs to be his focus, and it is his focus. That's what we're focusing on."