Raiders quarterback Derek Carr told Sirius XM Mad Dog Radio on Friday that Cooper's dramatic decline in production was due to playing on "one foot."
Cooper did have a career-high seven touchdown catches but he finished the 2017 campaign with only 48 receptions for 680 yards after surpassing 1,000 yards in each of his first two seasons.
"He would never say it to you -- no one would ever say it to you -- but I'll say it to you to have his back," Carr said on the radio show. "That man was out there playing as hard as he could and giving everything he could and, honestly, a lot of guys probably wouldn't have played with what he had."
Cooper had a three-game stretch from Weeks 3-5 where he had a combined four receptions for 23 yards. He did erupt for 11 catches for 210 yards and two touchdowns versus Kansas City on Oct. 19 but did not have another 100-yard game until the regular-season finale.
An ankle injury and concussion also forced the 2015 first-round pick (No. 4 overall) to miss a pair of games late in the season.