The Raiders' full squad opened training camp on Friday, and four players who were expected to be in uniform were not.
The club placed Rod Streater and Trent Richardson on the non-football injury list, while Stacy McGee and Trindon Holliday were added to the physically unable to perform list.
All four players can be activated at any time.
After registering 60 catches for 888 yards and four touchdowns over 14 starts in 2013, Streater's 2014 season ended prematurely after sustaining a foot injury Week 3 against the Patriots.
He is expected to be one of the Raiders' Top 3 receivers, along with rookie Amari Cooper and free agent signee Michael Crabtree.