Weston Hodkiewicz of Press-Gazette Media reports Barclay suffered a torn ACL on Tuesday in training camp, according to a league source.
Barclay was carted off the field after landing awkwardly on his right knee in practice. Barclay started 14 games last season in place of an injured Bryan Bulaga and was slated to be a backup at right tackle and both guard positions this season.
"Donny, they don't come any better than him," Packers coach Mike McCarthy said Tuesday. "He's exactly what you're looking for. If you want someone to emulate as a Green Bay Packer football players on and off the field, he's definitely one of your guys."