He was not being utilized in the offense
For whatever reason, Cooper had only two good games for the Raiders this season, and that was in an offense that was supposed to be much improved under the new coaching staff. Last season, the Raiders could have shrugged his poor year off on the fact they had an inexperienced offensive coordinator, but 2018 was mind-boggling at times.
Sure, he did have his moments, like when he went for 116 yards in Week 2 against the Denver Broncos, and 128 yards in Week 4 against the Cleveland Browns, but he has been invisible in the other four games. If you take away last week, due to the fact he left with a concussion, he still only hauled in four catches through the other three games.
In Dallas, maybe the Cowboys can get him turned around, but it was clear that he was not a big part in what the Raiders were trying to do on offense, no matter what Gruden had been saying. Cooper should have been an anchor on this offense for years to come, but for some reason, the last two offensive coordinators could get very little out of him.