Tackle Don Barclay, who was viewed by coach Mike McCarthy as the team's No. 6 offensive lineman, was carted from the practice field Tuesday afternoon with what appeared to be a significant right knee injury.
Barclay's right knee buckled beneath him while he was blocking rookie outside linebacker Adrian Hubbard during an 11-on-11 drill and the play ended with Barclay clutching his right knee in pain.
"I was standing right behind him," McCarthy said. "You (could) definitely see he was in a compromised position."
Barclay, who made the team as an undrafted free agent from West Virginia in 2012, wasn't projected as a starter this season, but started 21 games (including playoffs) the past two years, including 15 last season at right tackle.
With Barclay out, second-year guard Lane Taylor becomes the top reserve behind Josh Sitton and T.J. Lang.
Also, 2011 first-round pick Derek Sherrod, who suffered a career-threatening leg injury as a rookie and missed most of the past two seasons, becomes the top backup at tackle.
"Donny, they don't come any better than him. He's exactly what you're looking for," McCarthy said. "If you want someone to emulate what a Green Bay Packer football player is on and off the field, he's definitely one of your guys."
Barclay's injury comes less than a week after rookie fifth-round pick Jared Abbrederis tore the ACL in his right knee in practice on Wednesday.
Except in the case of the former University of Wisconsin wide receiver, he didn't realize the severity of the injury at the time, finishing practice Wednesday and practicing in full on Thursday before informing the medical staff that his knee didn't feel right. He's slated for surgery later this week or early next week.
"I didn't really think anything of it, really," said Abbrederis, who was expected to be part of the team's receiver rotation and was a candidate to be the punt returner. "When I got the news, I was kind of shocked."
The injuries to Barclay and Abbrederis come during a camp when the Packers are doing everything they can to limit injury risk - and have reduced the number of muscle pulls and strains that affected them in recent years.
The Packers have tapered down the intensity of each training camp practice, used GPS monitoring and other technology to track players' vital signs and exertion levels, and interrupted practice four times a day for extended water breaks called "regeneration periods."
McCarthy has been delivering on his promise to get to the bottom of why his team has been hurt by injuries more than any other over the past three years.
During McCarthy's tenure as coach, the Packers have averaged 53.3 starters games missed per season, fifth-most in the league over that span.
In hopes of reversing that trend, McCarthy has hired a director of performance nutrition to oversee the players' diets, moved the team's jog-through periods to the end of practices to allow players to taper down their exertion levels. And he has altered his in-season schedule, which the Packers are shifting to this week with their preseason opener at Tennessee on Saturday.
After wanting his players to be off their feet for 48 hours before kickoff each week, McCarthy will now have the team practice the day before games and not practice two days before games.
So this week, the Packers won't practice Thursday, will practice Friday and then play the Titans on Saturday.
"It's a philosophical change," McCarthy said at the start of camp. "This is the third year we've talked about (changing the schedule). I decided in spring to take the leap. We really just want to get that right and get our guys ready."