Warford, a third-round pick in the 2015 draft, suffered the injury "a while back," the source said. He is "well ahead" in his rehab from the injury, which was described as a huge gash on one of his knees, according to the source.
"He will be fine," another source with knowledge of the injury told ESPN.
The Raiders are said to be very high on Walford and are planning to expand his role in the passing game this season. The 6-foot-4, 250-pound Walford had 28 catches for 329 yards and three touchdowns last season.
Fox Sports first reported Tuesday that Walford suffered the injury.