The Pittsburgh Steelers placed tight end David Johnson, tight end Heath Miller, linebacker Sean Spence and nose tackle Alameda Ta'amu on the Physically Unable To Perform List (PUP) to begin training camp.
"I don't have any prognosis in terms of their availability or even when we'll start the process of considering those guys," said coach Mike Tomlin. "We'll take it day-by-day and monitor their progress."
Miller suffered MCL and ACL tears in his knee in Week 15 last season, but remains hopeful of being ready to play in time for the regular-season opener.
Johnson suffered a season-ending knee injury in the Steelers' preseason opener in 2012, and was re-signed in March.
Spence is recovering from a knee injury, while Ta'amu has a hamstring injury.
Players on the PUP list to begin training camp may be activated at any time.
"All others are ready to go," said Tomlin, who added that quarterback Ben Roethlisberger should not be limited by some knee discomfort. "We're excited to get started on good, even footing. I look forward to continuing with this development process."