Cooper was the third receiver started his career with back-to-back seasons with 1,000 yards receiving and 70 catches, following Odell Beckham Jr. and Marques Colston but has been unable to build on that success the past two years.
Jon Gruden continues to trade away the Oakland Raiders roster and now QB Dak Prescott gets another weapon on the offensive side of the ball. The Raiders are continuing to stockpile top picks in preparation for their move to Las Vegas.
The Raiders now own three No. 1 picks in the 2019 draft - their own, one from the Chicago Bears in the Mack deal and now the Cowboys' top pick. The quarterback has not played well under Gruden, and Gruden will be in a great position to draft a quarterback next year since he is armed with three first-round picks.
Cooper's signing does come with considerable risk. He's become a hit-or-miss option, ranking 68th in fantasy points per game among wideouts this season.
Gallup caught a 49-yard touchdown pass in Sunday's loss to Washington and has 10 catches for 190 yards.
Dallas receivers have just one 100-yard game combined through seven games, by Cole Beasley in a Week 6 win over Jacksonville.
The Cowboys made the playoffs once in two-plus seasons with Williams, in 2009, when they won the franchise's first postseason game since the last of five Super Bowl victories following the 1995 season.
Cooper is in the NFL's concussion protocol after suffering the injury October 14 in Oakland's loss to Seattle in London.