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 illness 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.
Richardson, the third overall pick in the 2012 draft, is hoping to revive his career in Oakland.
Once healthy, he will be fighting for carries with presumptive starter Latavius Murray and Roy Helu Jr.
On Monday, the Raiders placed Sio Moore on the Active/Physically Unable to Perform list.
He underwent offseason hip surgery.
Raiders added DT Stacy McGee and KR Trindon Holliday to PUP and placed WR Rod Streater and RB Trent Richardson on the NFI list— Scott Bair (@BairCSN) July 31, 2015
Non football injury list (got hurt away from team activities). Can be activated once they pass a physical https://t.co/e6a94UB1BO— Scott Bair (@BairCSN) July 31, 2015