Cooper, a Pro Bowl selection as a return man in 2017, was claimed off waivers on Wednesday by the Arizona Cardinals, who host the Rams on Sunday.
"They're getting a great competitor, a tough football player. He has great instincts when the ball is in his hands, a hard worker. We think the world of him here," Rams coach Sean McVay said in a conference call with media covering the Cardinals.
The Rams waived Cooper to open a spot to sign running back C.J. Anderson, who was added as insurance with Todd Gurley dealing with a knee injury.
"Really, it wasn't anything that Pharoh didn't do. It was more a result of just a numbers deal, where two of our three running backs on the active roster were banged up, and we had to create a spot. That's really a big added bonus, and I wish him nothing but the best," McVay said.
The wide receiver, selected by the Rams in the fourth round of the 2016 draft from South Carolina, was placed on injured reserve after he underwent ankle surgery following a Week 1 victory over the Raiders.
Cooper, who never had a significant presence in the Rams' passing game, averaged a league-best 27.4 yards on kick returns last season but has just 21.3 yards on 13 kickoffs this season.
McVay was asked if he is concerned that Cooper might share some of the Rams' secrets with the Cardinals.
"Yeah, there are certainly some different things that I'm sure Pharoh could share, but you never really know when we're going to do some of those different things. There's always an element of a player with knowledge of the system can be beneficial and helpful with regards to what you're trying to get done, but you just never know," he said.
ESPN's Josh Weinfuss contributed to this report.