Cooper was initially assigned the jersey number 19, but following the release of fellow receiver James Jones, he switched to number 89. Cooper made his NFL debut on September 13, 2015. He had five receptions for 47 yards in a 33–13 loss to the Cincinnati Bengals. In his second game against the Baltimore Ravens, he had 109 yards and caught his first touchdown on a 68-yard pass from Derek Carr. The following week, Cooper had a franchise rookie record eight receptions for 134 yards in a 27–20 road victory over the Cleveland Browns, becoming the first Raiders receiver with consecutive 100+ yard receiving games since Randy Moss in 2005. Through three games, his 290 receiving yards were third in NFL history. On November 8, 2015, Cooper passed Tim Brown's record for rookie receptions (43) in a 35–38 loss to the Pittsburgh Steelers.
