Oakland dealt Cooper to the Cowboys in October in exchange for Dallas' first-round pick in the 2019 NFL draft.
Cooper has exploded since the trade with 40 receptions for 642 yards and six touchdowns in six games as a member of the Cowboys.
In six games with the Raiders this season, Cooper had just 22 grabs for 280 yards and one score.
While Cooper struggled in Oakland this season and last season, Davis said he isn't surprised by the 24-year-old wideout's success in Dallas "because he was a great, great player."
Davis added, "We always had the fastest guy on the field no matter what. I think [my dad] would have loved Amari, but it just didn't work out in our system."
The Raiders selected Cooper with the No. 4 overall pick in the 2015 NFL draft out of Alabama.
He topped 1,000 yards and was named to the Pro Bowl in each of his first two seasons, but he seems to be coming into his own in Dallas.
Cooper has two games with 180 or more receiving yards as a member of the Cowboys, including last week's performance of 10 catches for 217 yards and three touchdowns in a 29-23 overtime win against the rival Philadelphia Eagles.
Quarterback Dak Prescott severely lacked a top-flight weapon in the passing game after Dez Bryant was released and Jason Witten retired during the offseason, but Cooper has undoubtedly turned things around in that regard.
Dallas is 5-1 since acquiring Cooper, and at 8-5, it holds a two-game lead in the NFC East and can win the division with victory over the Colts in Week 15
Meanwhile, the Raiders are just 3-10, and they are already looking ahead to the 2019 NFL draft, as they own three selections in the first round.